Bones, molecules, and crown- tetrapod origins...Schultze 2001). The monophyletic status of the...

Chapter 11

Bones, molecules, and crown-tetrapod origins

Marcello Ruta and Michael I. Coates

ABSTRACT

The timing of major events in the evolutionary history of early tetrapods is discussedin the light of a new cladistic analysis. The phylogenetic implications of this are com-pared with those of the most widely discussed, recent hypotheses of basal tetrapodinterrelationships. Regardless of the sequence of cladogenetic events and positionsof various Early Carboniferous taxa, these fossil-based analyses imply that the tetrapodcrown-group had originated by the mid- to late Viséan. However, such estimatesof the lissamphibian–amniote divergence fall short of the date implied by molecularstudies. Uneven rates of molecular substitutions might be held responsible for themismatch between molecular and morphological approaches, but the patchy qualityof the fossil record also plays an important role. Morphology-based estimates ofevolutionary chronology are highly sensitive to new fossil discoveries, the interpreta-tion and dating of such material, and the impact on tree topologies. Furthermore,the earliest and most primitive taxa are almost always known from very few fossillocalities, with the result that these are likely to exert a disproportionate influence.Fossils and molecules should be treated as complementary approaches, rather thanas conflicting and irreconcilable methods.

Introduction

Modern tetrapods have a long evolutionary history dating back to the Late Devonian.Their origins are rooted into a diverse, paraphyletic assemblage of lobe-finned bonyfishes known as the ‘osteolepiforms’ (Cloutier and Ahlberg 1996; Janvier 1996;Ahlberg and Johanson 1998; Jeffery 2001; Johanson and Ahlberg 2001; Zhu andSchultze 2001). The monophyletic status of the Tetrapoda and that of its majorconstituent clades – lissamphibians and amniotes – is supported by a large numberof morphological characters and by a wide range of molecular data (e.g. Duellmanand Trueb 1986; Panchen and Smithson 1987, 1988; Duellman 1988; Gauthier et al.1988a,b; Milner 1988, 1993; Hedges et al. 1990; Carroll 1991; Trueb and Cloutier1991; Hedges and Maxson 1993; Hay et al. 1995; Laurin and Reisz 1997, 1999; Fellerand Hedges 1998; Laurin 1998a–c; Hedges and Poling 1999; Pough et al. 2000). Theearly evolutionary history of the lissamphibian and amniote crown-groups has beenthe subject of detailed scrutiny (e.g. Laurin 1991; Laurin and Reisz 1995; Báez andBasso 1996; Lee 1995, 1997a,b; Rieppel and deBraga 1996; deBraga and Rieppel

TTEC11 05/06/2003 11:47 AM Page 224

Bones, molecules, and crown-tetrapod origins 225

1997; Gao and Shubin 2001), but the phylogenetic placement of some groups is notagreed upon, as exemplified by current debates about the position of turtles relativeto other amniotes (e.g. Reisz and Laurin 1991; Lee 1993, 1995, 1996, 1997a,b; Laurinand Reisz 1995; Rieppel and deBraga 1996; deBraga and Rieppel 1997; Platz andConlon 1997; Zardoya and Meyer 1998; Hedges and Poling 1999; Rieppel and Reisz1999; Rieppel 2000).

The last twenty years have witnessed a revived interest in early tetrapod inter-relationships. New discoveries and a refinement of phylogenetic techniques havebroadened our understanding of the anatomy and intrinsic relationships of severalgroups. Research in this field has had a significant impact on the shaping of thetetrapod stem-group (Lebedev and Coates 1995; Coates 1996; Ahlberg and Johanson1998; Johanson and Ahlberg 2001) and has led to the recognition of a previouslyunsuspected diversity of Mississippian taxa (e.g. Clack 1994, 1998a–d, 2001, 2002;Milner and Sequeira 1994; Rolfe et al. 1994; Smithson 1994; Smithson et al. 1994;Lombard and Bolt 1995; Clack and Finney 1997; Paton et al. 1999; Bolt andLombard 2000; Clack and Carroll 2000). However, the interrelationships of the vastmajority of Palaeozoic groups are still intensely debated. Lack of congruent resultsin the most widely discussed, recently published phylogenetic analyses is astonishing.Traditional views on the taxonomic memberships of the lissamphibian and amniotestem-groups (Bolt 1969, 1977, 1979, 1991; Heaton 1980; Panchen and Smithson 1987,1988; Milner 1988, 1993; Trueb and Cloutier 1991; Carroll 1995; Coates 1996;Lee and Spencer 1997; Sumida 1997; Clack 1998a–d; Paton et al. 1999) have beenchallenged repeatedly, notably in a series of recent papers by Laurin and Reisz (1997,1999), Laurin (1998a–c) and Laurin et al. (2000a,b). As a result, no consensushas emerged on the position of several groups relative to the lissamphibian–amniotephylogenetic split.

The ancestry of lissamphibians, as well as the status and mutual relationshipsof the three modern lissamphibian orders, are particularly controversial topics (seediscussions in Carroll and Currie 1975; Duellmann and Trueb 1986; Duellmann 1988;Bolt 1991; Milner 1988, 1993, 2000; Feller and Hedges 1998; Laurin 1998a–c; Carroll2000, 2001; Anderson 2001). Several authors have suggested that some or all of thelissamphibian orders are related to dissorophoids, a group of Permo-Carboniferousand Lower Triassic temnospondyls (e.g. Bolt 1969, 1977, 1979, 1991; Lombard andBolt 1979; Bolt and Lombard 1985; Milner 1988, 1990, 1993, 2000; Trueb and Cloutier1991; Boy and Sues 2000; Holmes 2000; Rocek and Rage 2000a,b; Yates andWarren 2000; Gardner 2001). However, much discussion centres on the identity ofthe immediate sister taxon to frogs, salamanders, and caecilians (also known asgymnophionans). The temnospondyl theory of lissamphibian origin has been revivedrecently by Carroll (2001) and Carroll and Bolt (2001). These authors hypothesizethat the ancestry of frogs and salamanders is rooted into two distinct families of dis-sorophoids, the amphibamids and branchiosaurids, respectively. Caecilians, however,are thought to be related to tuditanomorph microsaurs (one of the most diverse groupsof lepospondyls). In particular, the Lower Permian genus Rhynckonkos has beenregarded as the most derived stem-group gymnophionan (Carroll and Currie 1975;Carroll and Gaskill 1978; Milner 1993; Carroll 2000, 2001). In Laurin’s (1998a–c)and Laurin and Reisz’s (1997, 1999) analyses, temnospondyls are a plesion on thetetrapod stem-group (see also below), whereas lepospondyls form a paraphyletic array

TTEC11 05/06/2003 11:47 AM 25

226 Marcello Ruta and Michael I. Coates

of stem-lissamphibians. Within this paraphyletic array, lysorophids (long-bodied,Pennsylvanian to Lower Permian tetrapods characterized by a broad orbitotemporalfenestration; Wellstead 1991) are considered to be the nearest Palaeozoic relativesof crown-lissamphibians.

The evolutionary implications of alternative hypotheses of early tetrapod relation-ships will be considered elsewhere together with a new, comprehensive cladisticanalysis recently completed by the authors (Ruta et al., in press). A summary of theresults of this analysis (Figure 11.1) and a review of the chronology of major eventsin the evolutionary history of early tetrapods are presented here. We explore the implications of conflicting phylogenetic hypotheses on estimates of the time of divergence between lissamphibians and amniotes, and compare morphology-based ‘timetrees’ (for the use of this term, equivalent to Smith’s 1994, X-trees, see Hedges 2001)with those deriving from recent molecular analyses (e.g. Feller and Hedges 1998; Kumarand Hedges 1998; Hedges 2001). Several questions are addressed in this chapter:

(1) Do different morphology-based cladistic analyses of primitive tetrapods implydifferent chronological estimates of the separation between lissamphibians andamniotes, or the origin of the lissamphibian and amniote crown-groups?

(2) Are palaeontological and molecular time trees in serious conflict with each other,and what is the source of this conflict?

(3) What is the bearing of fossils on time tree reconstruction, especially whenintegrated with the results of molecular analyses?

Materials and methods

Which consensus for early tetrapods?

In a series of seminal papers, Smithson (1985), Panchen and Smithson (1987, 1988),Milner (1993), and Ahlberg and Milner (1994) discussed the pattern of character disribution in the apical part of the tetrapod stem-group and in the basal portion ofthe crown-group. A common feature of these studies is the separation of mostPalaeozoic tetrapods into two distinct lineages ultimately leading to lissamphibiansand amniotes. Several subsequent analyses (Carroll 1995; Lebedev and Coates 1995;Coates 1996; Clack 1998b,d; Paton et al. 1999) have supported the basal dichotomybetween these two clades (see Laurin 1998a, Laurin and Reisz 1999, Clack 2000, and Clack and Carroll 2000, for a historical perspective on the classification of earlytetrapods). Panchen and Smithson’s (1988) scheme of relationships is the most elo-quent example of a balanced cladogram (sensu Smith 1994): major tetrapod cladesare equally distributed on the ‘batrachomorph’ and ‘reptiliomorph’ branches of thecrown-group (equivalent to the lissamphibian and amniote stem-groups, respectively;see Coates 1996). According to Panchen and Smithson (1988), the evolutionary separation between lissamphibians and amniotes is a Late Devonian event, since theFamennian Ichthyostega appears as the least derived plesion on the lissamphibian stem-group. Other stem-lissamphibian plesions include, in crownward order, nectrideans(Bossy and Milner 1998), colosteids (Smithson 1982; Hook 1983; Godfrey 1989),microsaurs (Carroll and Gaskill 1978), and temnospondyls (Milner 1988, 1990, 1993).In Panchen and Smithson’s (1988) scheme, baphetids (Beaumont 1977; Beaumont and

TTEC11 05/06/2003 11:47 AM Page 226


=-

linea

ris

Oes

toce

ph

alu

s am

ph

ium

inu

mL

eth

iscu

s st

ock

iU

roco

rdyl

us

wan

des

ford

iiS

auro

ple

ura

pec

tin

ata

/ sca

lari

sP

tyo

niu

s m

arsh

iiD

iplo

cera

spis

bu

rkei

Dip

loca

ulu

s m

agn

ico

rnis

Dic

erat

osa

uru

s b

revi

rost

ris

Bat

rach

ider

pet

on

ret

icu

latu

mK

erat

erp

eto

n g

alva

ni

Sci

nco

sau

rus

cras

sus

Do

lich

op

arei

as d

isje

ctu

sA

del

og

yrin

us

sim

orh

ynch

us

Ad

elo

spo

nd

ylu

s w

atso

ni

Ach

ero

nti

scu

s ca

led

on

iae

Bra

chyd

ecte

s el

on

gat

us

/ new

ber

ryi

Od

on

terp

eto

n t

rian

gu

lare

Hyl

op

lesi

on

lon

gic

ost

atu

mM

icro

bra

chis

pel

ikan

iE

ury

od

us

pri

mu

sC

ard

ioce

ph

alu

s st

ern

ber

gi

Rh

ynch

on

kos

sto

valli

Pel

od

oso

tis

elo

ng

atu

mM

icra

rote

r er

yth

rog

eio

sH

apsi

do

par

eio

n le

pto

nS

axo

ner

pet

on

gei

nit

ziA

sap

hes

tera

inte

rmed

iaS

teg

otr

etu

s ag

yru

sP

anty

lus

cord

atu

sTu

dit

anu

s p

un

ctu

latu

sB

atro

pet

es f

rits

chia

Wes

tlo

thia

na

lizzi

ae

Pal

eoth

yris

aca

dia

na

Cap

torh

inu

s ag

uti

Lim

no

scel

is p

alu

dis

Dia

dec

tes

absi

tus

Pet

rola

cosa

uru

s ka

nse

nsi

s

Sey

mo

uri

a b

aylo

ren

sis/

san

juan

ensi

sD

isco

sau

risc

us

aust

riac

us

Ko

tlas

sia

pri

ma

So

len

od

on

sau

rus

jan

ensc

hi

Gep

hyr

ost

egu

s b

oh

emic

us

Bru

kter

erp

eto

n f

ieb

igi

Pro

tero

gyr

inu

s sc

hee

lei

Ph

olid

erp

eto

n s

cuti

ger

um

Eo

her

pet

on

wat

son

i

Ph

olid

erp

eto

n a

tth

eyi

Arc

her

ia c

rass

idis

caA

nth

raco

sau

rus

russ

elli

Cae

rorh

ach

is b

aird

i

basal node of stem-group amniotes

Val

do

trit

on

gra

cilis

Kar

auru

s sh

aro

viTr

iad

ob

atra

chu

s m

assi

no

tiE

oca

ecili

a m

icro

po

da

Alb

aner

pet

on

inex

pec

tatu

mS

cho

enfe

lder

pet

on

pre

sch

eri

Lep

toro

ph

us

ten

erA

pat

eon

ped

estr

isM

icro

mel

erp

eto

n c

red

ner

iD

ole

serp

eto

n a

nn

ecte

ns

Am

ph

ibam

us

gra

nd

icep

sE

osc

op

us

lock

ard

iP

laty

rhin

op

s ly

elli

Bro

iliel

lus

bre

vis

Eco

lso

nia

cu

tler

ensi

sP

ho

ner

pet

on

pri

cei

Ach

elo

ma

cum

min

siE

ryo

ps

meg

acep

hal

us

Den

dre

rpet

on

aca

dia

nu

m

Iso

dec

tes

ob

tusu

sB

alan

erp

eto

n w

oo

di

Trim

ero

rhac

his

insi

gn

isN

eld

asau

rus

wri

gh

tae

Ed

op

s cr

aig

i

Co

chle

osa

uru

s fl

ore

nsi

sC

hen

op

roso

pu

s le

wis

i

Eu

crit

ta m

elan

olim

net

esM

egal

oce

ph

alu

s p

ach

ycep

hal

us

Bap

het

es k

irkb

yiW

hat

chee

ria

del

tae

Cra

ssig

yrin

us

sco

ticu

sG

reer

erp

eto

n b

urk

emo

ran

iC

olo

steu

s sc

ute

llatu

sTu

lerp

eto

n c

urt

um

Ich

thyo

steg

a st

ensi

oei

Aca

nth

ost

ega

gu

nn

ari

Ven

tast

ega

curo

nic

aP

and

eric

hth

ys r

ho

mb

ole

pis

Eu

sth

eno

pte

ron

fo

ord

i

basal node of stem-group lissamphibians

crown-grouplissamphibians

crown-groupamniotes

Figure 11.1 Strict consensus of 60 most parsimonious trees derived from a PAUP* analysis (Rutaet al. in press). The stem-tetrapod and lissamphibian portions of the trees are in theupper half of the figure, the amniote portion in the lower half.

TTEC11 05/06/2003 11:47 AM Page 227


Smithson 1998; Milner and Lindsay 1998), anthracosauroids (Smithson 1985, 2000),seymouriamorphs (Laurin 2000), and diadectomorphs (Romer 1946; Heaton 1980;Berman et al. 1992, 1998; Lombard and Sumida 1992) are progressively more derivedstem-amniotes.

A series of key discoveries have been instrumental in redefining our concept of themost primitive tetrapods as well as in our understanding of the pattern of morpho-logical change at the ‘fish’–tetrapod transition (e.g. Coates and Clack 1990, 1991;Coates 1996; Jarvik 1996; Clack 1998b). As a result, the stem-tetrapod affinities ofmost Devonian taxa, including Acanthostega and Ichthyostega, are now universallyaccepted (but see Lebedev and Coates 1995, and Coates 1996, for a discussion of thepossible stem-amniote affinities of Tulerpeton). Regardless of the phylogenetic place-ment of Devonian taxa, comparisons between the most recent published phylogeniesreveal a drastic shift from dichotomously branching to pectinate tree topologies, imply-ing an increase in the number of stem-group branching events. The studies of Ahlbergand Milner (1994), Carroll (1995), Lebedev and Coates (1995), Coates (1996), Clack(1998b,d), and Paton et al. (1999) support Panchen and Smithson’s (1988) conclu-sions with regard to the basal dichotomy of Palaeozoic groups. These analyses tacklesuch diverse problems as the broad pattern of relationships between major tetrapodgroups (Carroll 1995), the reconstruction of the sequence of anatomical changes intaxa spanning the ‘fish’–tetrapod transition (Lebedev and Coates 1995; Coates 1996),and the placement of various problematic Mississippian tetrapods (e.g. Crassigyrinus,Whatcheeria, Eucritta) known to display a mixture of characters otherwise consideredto be unique to separate clades (Clack 1998b,d, 2000, 2001, 2002; Paton et al. 1999).

Laurin and Reisz’s (1997, 1999) and Laurin’s (1988a–c) analyses have cast doubton the deep separation of Palaeozoic tetrapods between lissamphibian-related andamniote-related taxa. Their cladograms suggest that several early tetrapods, such asCrassigyrinus, Tulerpeton, Whatcheeria, and baphetids, are equally closely relatedto lissamphibians and amniotes. These results challenge long-recognized patternsof character change and distribution near the base of the tetrapod crown-clade. Inparticular, traditional groups such as temnospondyls, embolomeres, gephyrostegids,and seymouriamorphs are regarded as discrete radiations preceding the lissamphibian–amniote phylogenetic split. The fossil membership of Laurin and Reisz’s (1997,1999) and Laurin’s (1988a–c) crown-group is smaller than in previous works.Importantly, lissamphibians now sit at the crownward end of a paraphyletic assem-blage of lepospondyls, in contrast with previous suggestions that the latter may forma highly diverse clade of stem-amniotes (Carroll 1995; but see also Carroll 2001).Anderson’s (2001) analysis agrees with Laurin and Reisz’s (1997, 1999) and Laurin’s(1988a–c) conclusions that lepospondyls are stem-lissamphibians (although onlyEocaecilia is used in Anderson’s work), and that seymouriamorphs, embolomeres,and temnospondyls (represented, respectively, by Seymouria, Proterogyrinus, and aclade consisting of Balanerpeton and Dendrerpeton) are progressively less derivedstem-tetrapod plesions. The diadectomorph Limnoscelis identifies the stem-amniotebranch of Anderson’s (2001) cladogram (Berman 2000; Clack and Carroll 2000, andreferences therein).

Very few early tetrapod groups have survived the intense phylogenetic reshufflingof recent analyses. Among those that have, diadectomorphs appear repeatedly asthe nearest relatives of crown-amniotes; likewise, the stem-tetrapod affinities of

TTEC11 05/06/2003 11:47 AM Page 228


colosteids and most Devonian forms have been retrieved consistently by different authors,despite differences in taxon sample size and the use of contrasting character order-ing, weighting, and coding regimes (see also Ruta et al., in press). These data suggest(although not conclusively) that some regions of the tetrapod tree are better corrob-orated and more stable than others (Panchen and Smithson 1987, 1988; Sumida andLombard 1991; Berman et al. 1992; Lombard and Sumida 1992; Sumida et al. 1992;Laurin and Reisz 1997, 1999; Lee and Spencer 1997; Sumida 1997; Berman et al.1998; Berman 2000, Laurin 1998a–c; Paton et al. 1999; Clack 2001).

Methodological note

The strict consensus topologies deriving from the most widely discussed publisheddata sets including Caerorhachis are considered here (Figures 11.2–11.7). The strictconsensus trees resulting from our new analysis (Figures 11.8–11.9) and fromexperiments of character removal (Figure 11.10) are also illustrated. As in Rutaet al.’s (2001) paper, Lebedev and Coates’ (1995) and Clack’s (1998b, d) analyseshave been omitted, since they are superseded by Coates’ (1996) and Paton et al.’s (1999)works, respectively. Strict consensus trees are plotted on a stratigraphical scaleresolved down to stage level (geological timescale based on Briggs and Crowther, 2001,and references therein). For simplicity, stages are drawn to the same length, andnot proportional to their actual duration, although dates in millions of years beforepresent (Ma) are appended, where possible, to stage names. In addition, the knownranges of major early tetrapods groups are used (Benton 1993), instead of specificoccurrences of individual species. The use of whole ranges permits rapid and easy com-parisons between tree shapes, and circumvents the problem of comparing time treesbuilt on different taxon samples for each group. Internodes within monophyletic groupsare represented by vertical bars of fixed, arbitrary length (except where ghost rangesare present; Smith 1994). This length represents merely a graphical expedient and doesnot imply an equal time for the origin of adjacent nodes. It has, however, the incon-venient effect of generating chronologically ‘deep’ origin events for some groups, depend-ing upon the number of internodes and the placement of the stratigraphically oldestmembers of a group. Since the actual time occurring between adjacent nodes is unknown,the age of a node leading to two sister taxa is conservatively taken to coincide withthe age of the older taxon.

Where species or genera are used as Operational Taxonomic Units (OTUs), it ispossible to identify the point of divergence between sister groups, even if wholestratigraphical ranges are employed. For example, in Anderson’s (2001) tree, the stem-caecilian Eocaecilia micropoda is the sister taxon to brachystelechid microsaurs.Therefore, the divergence of caecilians can be graphically plotted within the strati-graphical range of microsaurs instead of at the base of such a range (Figure 11.7).Paraphyletic groups pose problems when whole ranges are used. A possible way aroundthis consists of splitting the ranges of large groups into the smaller ranges in whichtheir component subgroups occurred. For simplicity, however, only total ranges are employed here, whereas paraphyletic groups are denoted by names in invertedcommas.

With regards to Coates’ (1996) analysis, Ruta et al. (2001) pointed out that introduction of corrected scores for digit number and coronoid fangs in some taxa

TTEC11 05/06/2003 11:47 AM Page 229


Aca

nth

ost

ega

gu

nn

ari

Ich

thyo

steg

a st

ensi

oei

Eu

sth

eno

pte

ron

fo

ord

iP

and

eric

hth

ys r

ho

mb

ole

pis

Cae

rorh

ach

is b

aird

i

EM

BO

LO

ME

RI

NE

CT

RID

EA

AÏS

TOP

OD

A

DIA

DE

CTO

MO

RP

HA

cro

wn

-gro

up

AM

NIO

TA

Wes

tlo

thia

na

lizzi

ae

SE

YM

OU

RIA

MO

RP

HA

MIC

RO

SA

UR

IA

LYS

OR

OP

HIA

AD

EL

OS

PO

ND

YL

I

BA

PH

ET

IDA

E

Cra

ssig

yrin

us

sco

ticu

s

TE

MN

OS

PO

ND

YL

I

CO

LO

ST

EID

AE

basal node of thetetrapod crown-group

Lochkovian

Pragian

Emsian

Eifelian

Givetian

Frasnian

Famennian

Tournaisian

Visean

Serpukhovian

Bashkirian

Moscovian

Kazimovian

Gzhelian

Asselian

Sakmarian

Artinskian

Kungurian

Roadian

Wordian

Capitanian

Wuchiapigian

Changhsingian

Induan

Olenekian

Anisian

Ladinian

Carnian

Norian

Rhetian

Hettangian

Sinemurian

Pliensbachian

Toarcian

Aalenian

Bajocian

Bathonian

Callovian

Oxfordian

Kimmeridgian

Tithonian

Berriasian

Valanginian

Hauterivian

Barremian

Aptian

Albian

Cenomanian

Turonian

Coniacian

Santonian

Campanian

Maastrichtian

Danian

Thanetian

Ypresian

Lutetian

Barthonian

Priabonian

Rupelian

Chattian

Aquitanian

Burdigalian

Langhian

Serravallian

Tortonian

Messinian

Zanclean

Piacenzian

DE

VO

NIA

NC

AR

BO

NIF

ER

OU

SP

ER

MIA

NT

RIA

SS

ICJU

RA

SS

ICC

RE

TAC

EO

US

EARLY

MIDDLE

LATE

CISURALIAN

PAL

EO

GE

NE

NE

OG

EN

E

GUADALUPIAN

LOPINGIAN

PENNSYLVANIAN

MISSISSIPPIAN

EARLY

MIDDLE

LATE

LATE

MIDDLE

EARLY

EARLY

LATE

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENE

Gelasian

410

400390380375370355

345

325

295

250

240233230220

20320019118417517016416015414614113513112311711310896928887837265

5346403733.72823.520.315.814.3117.35.33.4

1.75

Figure 11.2 Carroll’s (1995) analysis plotted on a timescale.

TTEC11 05/06/2003 11:47 AM Page 230


causes loss of phylogenetic resolution among the majority of post-panderichthyidDevonian tetrapods (Tulerpeton appears as a basal stem-amniote; Figure 11.3). It can be shown, however, that such poor resolution is due exclusively to the unstableposition of Metaxygnathus and Ventastega. The branching sequence of remainingDevonian taxa is the same as that retrieved by Coates (1996).

Ahlberg and Clack’s (1998) analysis (Figure 11.4) poses additional difficulties, becauseseveral traditional groups (notably anthracosaurs and temnospondyls) appear aspolyphyletic, overlapping arrays of taxa (comments in Ruta et al. 2001). However,an expanded version of their dataset (see Appendix 11.1 for a list of the new char-acters added and their description) retrieves traditional groups after a reweighted run.The strict consensus of the resulting five equally parsimonious trees is discussedhere (Figure 11.11), instead of Ahlberg and Clack’s original consensus cladogram. Asingle origin for anthracosaurs and temnospondyls is obtained after analysing theirexpanded matrix. Lower jaw data can be shown to carry phylogenetic signal in derivedportions of the tetrapod tree as well as in the crownward portion of the stem-group.However, the degree to which this signal matches that yielded by other characters is,at present, difficult to evaluate. The impact of lower jaw characters on tree topologymust await exhaustive treatment of additional data (e.g. Bolt and Lombard 2001) codedfor a larger number of taxa.

Definition and content of Tetrapoda

Any fossil taxon that can be shown, based on a formal character analysis, to be phylogenetically more closely related to extant lissamphibians and amniotes thanto any other extant monophyletic group is, by definition, a stem-group tetrapod. If afossil taxon is more closely related to either lissamphibians or amniotes, it is acrown-group tetrapod (Hennig 1966; Jefferies 1979; Craske and Jefferies 1989).Justification in support of a total-group (or stem-based) clade Tetrapoda will be pro-vided elsewhere (Ruta et al., in press; but see discussions in Ahlberg and Clack 1998,Laurin et al. 2000a,b, and Anderson 2001 for alternative nomenclatural solutions).

Briefly, we do not advocate an apomorphy-based definition of Tetrapoda that excludesthe ‘fish-like’ portion of the tetrapod stem-group (Ahlberg and Clack 1998), nor dowe restrict the name Tetrapoda to the crown-clade (Laurin 1998a). Instead, wefavour an operational definition (Coates et al. 2000), whereby all taxa that belongin the total-group of the extant clade Tetrapoda, but which are not members of thecrown-group, are simply referred to as stem-group tetrapods (see also Budd 2001;Jeffery 2001).

Fossil evidence for the origin of crown-tetrapods

With few exceptions, published analyses postulate that the origin of the tetrapod crown-group had occurred by the mid- to late Viséan (e.g. Paton et al. 1999). A Late Devoniandivergence between lissamphibians and amniotes was first proposed by Panchen andSmithson (1987, 1988), who interpreted Ichthyostega as a basal stem-group lissam-phibian (see also above). Lebedev and Coates (1995) and Coates (1996) also suggestedthat the origin of the tetrapod crown-group was a Late Devonian event, but in thiscase, the hypothesized divergence time was based upon their interpretation of the

TTEC11 05/06/2003 11:47 AM Page 231


Lochkovian

Pragian

Emsian

Eifelian

Givetian

Frasnian

Famennian

Tournaisian

Visean

Serpukhovian

Bashkirian

Moscovian

Kazimovian

Gzhelian

Asselian

Sakmarian

Artinskian

Kungurian

Roadian

Wordian

Capitanian

Wuchiapigian

Changhsingian

Induan

Olenekian

Anisian

Ladinian

Carnian

Norian

Rhetian

Hettangian

Sinemurian

Pliensbachian

Toarcian

Aalenian

Bajocian

Bathonian

Callovian

Oxfordian

Kimmeridgian

Tithonian

Berriasian

Valanginian

Hauterivian

Barremian

Aptian

Albian

Cenomanian

Turonian

Coniacian

Santonian

Campanian

Maastrichtian

Danian

Thanetian

Ypresian

Lutetian

Barthonian

Priabonian

Rupelian

Chattian

Aquitanian

Burdigalian

Langhian

Serravallian

Tortonian

Messinian

Zanclean

Piacenzian

DE

VO

NIA

NC

AR

BO

NIF

ER

OU

SP

ER

MIA

NT

RIA

SS

ICJU

RA

SS

ICC

RE

TAC

EO

US

EARLY

MIDDLE

LATE

CISURALIAN

PAL

EO

GE

NE

NE

OG

EN

E

GUADALUPIAN

LOPINGIAN

PENNSYLVANIAN

MISSISSIPPIAN

EARLY

MIDDLE

LATE

LATE

MIDDLE

EARLY

EARLY

LATE

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENE

Gelasian

410

400390380375370355

345

325

295

250

240233230220

20320019118417517016416015414614113513112311711310896928887837265

5346403733.72823.520.315.814.3117.35.33.4

1.75

Eu

sth

eno

pte

ron

fo

ord

i

Hyn

erp

eto

n b

asse

tti

MIC

RO

SA

UR

IA

TE

MN

OS

PO

ND

YL

I

CO

LO

ST

EID

AE

Aca

nth

ost

ega

gu

nn

ari

Ven

tast

ega

curo

nic

a

Ich

thyo

steg

a st

ensi

oei

Elg

iner

pet

on

pan

chen

i


Tule

rpet

on

cu

rtu

m

Cae

rorh

ach

is b

aird

i

EM

BO

LO

ME

RI

Cra

ssig

yrin

us

sco

ticu

sW

hat

chee

ria

del

tae

Wes

tlo

thia

na

lizzi

ae

Met

axyg

nat

hu

s d

enti

culu

s

Pan

der

ich

thys

rh

om

bo

lep

is

Figure 11.3 Coates’ (1996) analysis plotted on a timescale.

TTEC11 05/06/2003 11:47 AM Page 232


Pan

der

ich

thys

rh

om

bo

lep

is

Cae

rorh

ach

is b

aird

i

EM

BO

LO

ME

RI

GE

PH

YR

OS

TE

GID

AE

MIC

RO

SA

UR

IA

SE

YM

OU

RIA

MO

RP

HA

BA

PH

ET

IDA

E

TE

MN

OS

PO

ND

YL

I

Ich

thyo

steg

a st

ensi

oei

Cra

ssig

yrin

us

sco

ticu

s

Wh

atch

eeri

a d

elta

eC

OL

OS

TE

IDA

E

NE

CT

RID

EA

cro

wn

-gro

up

AM

NIO

TA

Tule

rpet

on

cu

rtu

m

Aca

nth

ost

ega

gu

nn

ari

Ven

tast

ega

curo

nic

a

Met

axyg

nat

hu

s d

enti

culu

s

Ob

ruch

evic

hth

ys g

raci

lisE

lgin

erp

eto

n p

anch

eni


?

?

Lochkovian

Pragian

Emsian

Eifelian

Givetian

Frasnian

Famennian

Tournaisian

Visean

Serpukhovian

Bashkirian

Moscovian

Kazimovian

Gzhelian

Asselian

Sakmarian

Artinskian

Kungurian

Roadian

Wordian

Capitanian

Wuchiapigian

Changhsingian

Induan

Olenekian

Anisian

Ladinian

Carnian

Norian

Rhetian

Hettangian

Sinemurian

Pliensbachian

Toarcian

Aalenian

Bajocian

Bathonian

Callovian

Oxfordian

Kimmeridgian

Tithonian

Berriasian

Valanginian

Hauterivian

Barremian

Aptian

Albian

Cenomanian

Turonian

Coniacian

Santonian

Campanian

Maastrichtian

Danian

Thanetian

Ypresian

Lutetian

Barthonian

Priabonian

Rupelian

Chattian

Aquitanian

Burdigalian

Langhian

Serravallian

Tortonian

Messinian

Zanclean

Piacenzian

DE

VO

NIA

NC

AR

BO

NIF

ER

OU

SP

ER

MIA

NT

RIA

SS

ICJU

RA

SS

ICC

RE

TAC

EO

US

EARLY

MIDDLE

LATE

CISURALIAN

PAL

EO

GE

NE

NE

OG

EN

E

GUADALUPIAN

LOPINGIAN

PENNSYLVANIAN

MISSISSIPPIAN

EARLY

MIDDLE

LATE

LATE

MIDDLE

EARLY

EARLY

LATE

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENE

Gelasian

410

400390380375370355

345

325

295

250

240233230220

20320019118417517016416015414614113513112311711310896928887837265

5346403733.72823.520.315.814.3117.35.33.4

1.75

Figure 11.4 Ahlberg and Clack’s (1998) analysis plotted on a timescale. The two arrows point tothe positions of the basal node of the tetrapod crown-group based upon the derivationof lissamphibians from a lepospondyl or a temnospondyl ancestor.

TTEC11 05/06/2003 11:47 AM Page 233


Famennian Tulerpeton curtum (Lebedev 1984) as a stem-group amniote (see also Clackand Carroll 2000). Tulerpeton has been neglected in most recent analyses, despite thefact that it is known from well preserved, although incomplete, postcranial material(see Lebedev and Clack 1993 and Ahlberg and Clack 1998 for a discussion of cranialand lower jaw elements attributed to this taxon). However, subsequent studies(e.g. Ahlberg and Clack 1998; Clack 2002; Ruta et al., in press) concur in assigningTulerpeton, as well as all other Devonian taxa, to the tetrapod stem-group.Nevertheless, we acknowledge that hypotheses of a Late Devonian separation betweenlissamphibians and amniotes are consistent with some recent molecular analyses (e.g.Kumar and Hedges 1998; Hedges 2001).

Mississippian tetrapods are rare. Incomplete remains from the mid-Tournaisian siteof Horton Bluff, Nova Scotia (Clack and Carroll 2000), are the oldest documentedexamples, but these specimens cannot be diagnosed unambiguously as ‘batra-chomorph’ or ‘reptiliomorph’. Some isolated humeri appear to be morphologicallyintermediate between those of Tulerpeton (Lebedev and Coates 1995) and the‘anthracosauroid’ Eoherpeton (Smithson 1985; Clack and Carroll 2000), whileothers are more similar to colosteid humeri (Godfrey 1989). Additional specimensinclude femora as well as endochondral and dermal shoulder girdle elements. Recentlydiscovered, mid-Viséan remains from central Queensland represent the only recordof Carboniferous tetrapods from East Gondwana (Thulborn et al. 1996). Althoughfragmentary, this fauna is thought to include the earliest known representativesof colosteids and ‘anthracosauroids’ (fide Thulborn et al. 1996; Clack and Carroll2000).

The next oldest Mississippian record is represented by a Whatcheeria-like animalfrom the late Tournaisian of Scotland (Clack and Finney 1997). Like Whatcheeria(Lombard and Bolt 1995; Bolt and Lombard 2000), the new tetrapod reveals anarray of ‘reptiliomorph’, ‘batrachomorph’, and primitive features. The manual char-acter analysis of Lombard and Bolt (1995) and the computer-assisted analyses of Coates(1996), Clack (1998b,d), and Paton et al. (1999) concur in assigning Whatcheeria tothe basal portion of the ‘reptiliomorph’ branch of the tetrapod tree. Certain recent,comprehensive analyses (Laurin and Reisz 1997, 1999; Laurin 1998a–c; Anderson2001) have ignored Whatcheeria. Other studies, including Ahlberg and Clack’s (1998)and our own (Ruta et al., in press), suggest that Whatcheeria is a stem-tetrapod.The exceptional preservation and abundant material of Whatcheeria provide animportant data source for comparative anatomical and phylogenetic studies of earlytetrapods. The sequence of branching events in the crownward part of the tetrapodstem-group is the subject of much current debate and may ultimately lead to are-assessment of the polarity of several characters. A detailed study of Whatcheeriaand the new Whatcheeria-like animal from the Scottish Tournaisian will certainly proveto be crucial in this respect.

Casineria kiddi, a 340 million years old, incomplete skeleton from Gullane, CheeseBay, Scotland (lower part of late Viséan), is the next animal to be considered.Regarded as the earliest undisputed amniote, it is the oldest tetrapod showing apentadactyl forelimb, and predates the uppermost Viséan fauna from the Scottish siteof East Kirkton (see below). Its relatively low, squared off neural spines are reminis-cent of those of such primitive ‘anthracosauroids’ as Silvanerpeton and Eldeceeon (Clack1994; Smithson 1994), whereas its long, curved ribs, separate scapular and coracoid

TTEC11 05/06/2003 11:47 AM Page 234


ossifications and proportions of the manus are similar to those of certain embolom-eres and various basal crown-group amniotes. However, different combinations ofthese features have also been observed in other taxa, such as certain microsaurs andWhatcheeria (Carroll and Gaskill 1978; Lombard and Bolt 1995; Bolt and Lombard2000). Although Paton et al.’s (1999) cladistic analysis identified Casineria as a basalamniote, it is noteworthy that it failed to resolve its position relative to such diversetaxa as Westlothiana, Captorhinus, Petrolacosaurus, and Paleothyris (Ruta et al.,in press).

Several other taxa with possible ‘reptiliomorph’ affinities have been included in ouranalysis. Westlothiana lizziae from East Kirkton is usually regarded as one of the mostprimitive stem-group amniotes (Smithson 1989; Smithson and Rolfe 1990; Smithsonet al. 1994). However, Laurin and Reisz (1999) placed this taxon as the closest out-group to the tetrapod crown-clade. Conversely, our analysis strengthens Smithsonet al.’s (1994) conclusions and offers a novel perspective for interpreting the puzzlingmixture of ‘lepospondyl’ as well as basal amniote features in Westlothiana (see alsothe analysis in Anderson 2001). Caerorhachis bairdi, probably from the lowermostSerpukhovian of Scotland, was originally described as a basal temnospondyl (Holmesand Carroll 1977), but has been reinterpreted as a basal stem-amniote by Ruta et al.,in press (see also discussion in Milner and Sequeira 1994; Coates 1996).

The late Viséan Crassigyrinus scoticus has been the subject of controversy eversince its discovery. Panchen (1985) and Panchen and Smithson (1990) redescribedits cranial and postcranial anatomy. Panchen and Smithson (1988) placed it on the‘reptiliomorph’ branch of their tetrapod cladogram, either as sister taxon to ‘anthra-cosauroids’, or as sister taxon to a clade encompassing ‘anthracosauroids’ and sey-mouriamorph as sister group to diadectomorphs plus crown-group amniotes. Furtherpreparation of the material resulted in a reassessment of the morphology of the palate(Clack 1996), snout, and skull roof (Clack 2000) leading to the recognition of an arrayof plesiomorphic features. Some recent analyses (Coates 1996; Clack 1998b,d; Patonet al. 1999), place Crassigyrinus as a basal embolomere, whereas Laurin and Reisz(1997, 1999), Ahlberg and Clack (1998) and Laurin (1998a–c) identify it as a crown-ward stem-tetrapod. The latter conclusion is also supported by Ruta et al. (in press).

The nature of the conflict

From the account above, it is clear that the base of the tetrapod crown-group hasuncertain boundaries. In fact, only the amniote affinities of Casineria (see also below)remain uncontroversial. The existence of incongruent tree topologies is due to severalcauses that are not mutually exclusive. The use of incomplete or poorly preservedtaxa is likely to result in multiple, equally parsimonious solutions. While implicitlyassumed in all fossil-based studies, the influence of such taxa on cladogram topologyremains largely unexplored (but see Wilkinson 1995; Anderson 2001; Kearney 2002).However, as demonstrated by Coates (1996), incomplete taxa (e.g. Hynerpeton;Daeschler et al. 1994) do not necessarily behave as ‘rogue’ OTUs. Sometimes, the pres-ence of just one unambiguous synapomorphy is sufficient to stablize the affinities offragmentary material.

Another potential source of character conflict is the fact that various taxa sharingfeatures with two or more different groups deliver confounding signals. In simple cases,

TTEC11 05/06/2003 11:47 AM Page 235


variations in the taxon sample are likely to affect the outcome of an analysis through‘attraction’ of such ‘chimaera’-like taxa. However, the effects of taxon and/or character deletions/inclusions are not predictable. In those cases in which an optimal‘balance’ of taxa and characters is achieved, the position of key fossils may remainunresolved. Clack’s (2001) analysis provides an excellent example of this taxon/character interplay. Specifically, a clade consisting of Eucritta and baphetids formsa trichotomy with temnospondyls and a diverse group including Crassigyrinus,Whatcheeria, gephyrostegids, and embolomeres (but see also Clack 1998a) in the twoequally parsimonious trees discussed by Clack (2001).

Several groups of early tetrapods are so specialized that they provide little or noindication as to their possible ancestry or sister group. Carroll (2001) has emphasizedthis observation repeatedly, identifying the apparent excess of apomorphies andwidespread homoplasy as responsible for obscuring relationships among basalcrown-group tetrapods. However, while homoplasy might be widespread, we thinkit unlikely that the current tetrapod database contains insufficient phylogenetic signal.Thus, a quick inspection of published analyses reveals that the structure of severalmatrices is not random. A comparison between two of the most comprehensivedatasets – Carroll’s (1995) and Laurin and Reisz’s (1999) – serves to illustrate this point.Despite the use of different taxon and character samples, Carroll’s (1995) and Laurinand Reisz’s (1999) cladograms are mostly congruent. Crown-lissamphibians are placedamong lepospondyls in Laurin and Reisz’s study, but are excluded from Carroll’sanalysis. If lissamphibians are not taken into account, the sequences of branching eventsin Carroll’s (1995) and Laurin and Reisz’s (1999) tree topologies are remarkablysimilar. Minor differences concern the mutual relationships of the lepospondyl orders,the position of Westlothiana (grafted to a diadectomorph–amniote clade in Carroll’sanalysis, but sister taxon to a diadectomorph–amniote–lepospondyl clade in Laurinand Reisz’s) and the pattern of sister group relationships between baphetids, colosteids,and temnospondyls (all three groups branch from adjacent nodes in both analyses).It is also noteworthy that (excluding Laurin and Reisz’s location of lepospondyls) thebranching sequence in the basal stretch of the putative stem-amniote groups (e.g.embolomeres, gephyrostegids, seymouriamorphs, Westlothiana) resembles that pro-posed by several earlier authors (e.g. Lombard and Sumida 1992; Smithson et al. 1994;Lee and Spencer 1997; Sumida 1997).

Results

A new analysis for early tetrapods

Recent advances in our knowledge of early tetrapod anatomy have contributed toan expanded and refined database (Trueb and Cloutier 1991; Coates 1996; Laurinand Reisz 1997, 1999; Ahlberg and Clack 1998; Clack 1998b; Laurin 1998a–c;Lombard and Bolt 1999; Paton et al. 1999; Bolt and Chatterjee 2000; Schoch andMilner 2000; Yates and Warren 2000; Bolt and Lombard 2001). In our analysis,we have sought to use the maximum practical range of taxon exemplars, consistentwith methodological arguments arising from a series of recent studies (Nixon andDavis 1991; Anderson 2001; Prendini 2001; Salisbury and Kim 2001; Ruta et al.in press).

TTEC11 05/06/2003 11:47 AM Page 236


The new data matrix encompasses 90 tetrapod species coded for 213 cranial and94 postcranial characters. The results support the hypothesis of a deep evolutionarysplit between stem-lissamphibians and stem-amniotes. Further major features ofthese results are summarized as follows (Figure 11.1):

(1) The post-panderichthyid part of the tetrapod stem-group includes, in crownwardorder, Ventastega curonica, Acanthostega gunnari, Ichthyostega stensioei,Tulerpeton curtum, Colosteidae, Crassigyrinus scoticus, Whatcheeria deltae,and Baphetidae.

(2) Caerorhachis bairdi, embolomeres, gephyrostegids, Solenodonsaurus janenschi,seymouriamorphs, a clade consisting of Westlothiana lizziae plus lepospondyls,and diadectomorphs are progressively more crownward stem-amniotes.

(3) Within lepospondyls, microsaurs are paraphyletic relative to lysorophids,adelospondyls (including Acherontiscus), and a clade encompassing nectrideansplus aïstopods.

(4) Eucritta melanolimnetes is basal to temnospondyls, which form a paraphyleticarray of taxa relative to crown-lissamphibians.

(5) Albanerpetontids and a diverse dissorophoid clade consisting of branchiosaurids,micromelerpetontids, and amphibamids are successively more outlying sistergroups of crown-lissamphibians.

(6) Caecilians are the sister group to a salientian–caudate clade.

The tetrapod crown-group is bracketed at its base by Eucritta and Caerorhachis,a pair of Scottish taxa noted for their mixture of features otherwise considered tobe characteristic of such different groups as temnospondyls, baphetids, and ‘anthra-cosauroids’ (Clack 1998b, 2001; Ruta et al. 2001). A comprehensive treatment of thecharacters and results of the new analysis is presented elsewhere (Ruta et al. in press).PAUP* 4.0b10 (Swofford 1998; see Ruta et al., in press for details of the searchsettings used) finds 60 shortest trees at 1303 steps. If Casineria (Paton et al. 1999)and Silvanerpeton (Clack 1994) are included in the analysis, then a strict consensusof the resultant 120 equally parsimonious trees shows considerable loss of resolutionin the basal part of the amniote stem-group. The polytomy subtends Casineria,Silvanerpeton, embolomeres, gephyrostegids, Solenodonsaurus, Discosauriscus,Kotlassia, and Seymouria. However, an agreement subtree shows that Silvanerpetonbranches from the amniote stem between Caerorhachis and embolomeres (see alsoClack 1994), but that Casineria is a ‘rogue’ taxon. Despite its uncertain placement,Casineria emerges, consistently, as a stem-amniote, in partial agreement with Patonet al.’s (1999) conclusions.

The new analysis supports traditional views on the amniote affinities of‘anthracosaurs’, seymouriamorphs, and diadectomorphs (Panchen and Smithson1987, 1988; Lombard and Sumida 1992; Lee and Spencer 1997; Sumida 1997), andidentifies temnospondyls as a paraphyletic grade group on the lissamphibian stem (Milner1988, 1990, 1993, 2000). The general results resemble most closely those obtainedby Carroll (1995), especially with regards to the monophyly of lepospondyls andtheir placement on the amniote stem. We are currently evaluating the nature of thelepospondyl groups and the degree of support (morphological as well as statistical)assigned to various nodes within this assemblage (see also Ruta et al. in press). Thus,

TTEC11 05/06/2003 11:47 AM Page 237


while the position of microsaurs on the amniote stem-group is also retrieved in exper-iments of taxon and/or character deletion and reweighting, the placement of remain-ing lepospondyls can be affected drastically. For instance, when post-cranial dataare omitted from the analysis, the relationships of remaining lepospondyls changesignificantly: they are relocated as stem-group tetrapods, as sister group to colosteids.Similar results are obtained if nectrideans and lysorophids are excluded from thedataset. In this case, aïstopods are paired with adelospondyls and, together, they formthe sister group to colosteids. The evolutionary implications of these results haveyet to be explored in depth. Carroll (1999) has suggested that similarities betweenlepospondyls and primitive amniotes (especially in the configuration of the vertebrae)represent convergent features related to precocious ossification attained at a small bodysize. However, the stem-amniote position of microsaurs is not affected by deletion ofpostcranial characters (Figure 11.10). It is possible that lepospondyl monophyly in theoriginal analysis results from the cumulative effect of implied reversals and optimizationsof missing entries related to cranial and postcranial features. Further work in this areais needed.

The results match those of certain previous studies, especially with regards to theposition of lissamphibians and the branching pattern in the basal part of the amniotestem. This is unsurprising, because the matrix includes, so far as possible, the major-ity of characters used in previous analyses (details in Ruta et al. in press), as well asfurther data from smaller morphological sets (e.g. Trueb and Cloutier 1991). As anadditional test of the performance of character subsets, we excluded lower jaw data.Removal of these has no major effect on the overall tree topology. The latter matchesthe results retrieved in the original analysis, except that crown-lissamphibians aremore deeply nested in the derived portion of the temnospondyl tree, whereas mosttuditanomorphs are collapsed in a large polytomy. We conclude that cranial and postcra-nial characters are not in conflict with lower jaw data (but see discussion in Ahlbergand Clack 1998).

Elsewhere (Coates et al. 2000), we pondered a few of the biological implicationsof taxon rearrangements in Laurin’s (1998a–c) preferred tree topology, in whichlysorophids are the hypothesized closest relatives to frogs, salamanders, and caecil-ians. We concur with Carroll (2001) and Carroll and Bolt (2001) that hardly anyfeature of crown-lissamphians can be identified as a convincing synapomorphyshared uniquely by lysorophids with each of the three lissamphibian orders. However,Laurin et al. (2000b) correctly point out that grafting lissamphibians to temno-spondyls is a much worse fit for their data than the topology retrieved from earlieranalyses (e.g. Laurin 1998a–c). Prompted by Laurin et al.’s (2000b) suggestion thatadditional phylogenetic analyses should be performed to test the origin of lissamphibians,we have added characters that have been proposed previously as putative sharedfeatures of temnospondyls and lissamphibians (e.g. Bolt 1969, 1977, 1979, 1991; Milner1988, 1990, 1993, 2000; Trueb and Cloutier 1991; Gardner 2001; Ruta et al., inpress). Our analysis favours dissorophoids as the closest relatives of lissamphibiansamong the vast array of Palaeozoic tetrapods.

A recent study by Yeh (2002) on the effect of miniaturization on the skeleton offrogs has shown that, although paedomorphosis is responsible for the loss of severalskull bones in miniaturized vertebrates, there is no simple correlation between suchlosses and small size. However, several bones that ossify late during development, such

TTEC11 05/06/2003 11:47 AM Page 238


as quadratojugals, columellae, and palatines, are also those that are lost most frequently.In most anurans, such bones are usually post-metamorphic. Therefore, their loss isplausibly linked to paedomorphosis. In addition, miniaturization may affect membersof the same clade in profoundly different ways. Interestingly, the medial skull elementsof miniaturized frogs (e.g. parasphenoid) are transversely expanded, whereas the lateral elements (e.g. pterygoids) are laterally compressed. Certain bones are shortened in comparison with their homologues in non-miniaturized frogs (e.g. maxilla, quadratojugal, vomer). Several of these features are also recorded in certaindissorophoids. Striking similarities between the ontogenetic changes in the skull ofvarious modern lissamphibians and those of amphibamids and branchiosaurids addstrength to the temnospondyl hypothesis of lissamphibian ancestry (Milner 1988, 1990,1993, 2000; Schoch 1992, 1995, 1998; Boy and Sues 2000; Carroll 2001). The listof ‘absence’ features that link lysorophids to lissamphibians in Laurin and Reisz’s (1997,1999) and Laurin’s (1998a–c) analyses calls for a cautious treatment of character lossesand characters associated with small size. As noted by Milner (1988), examples ofconvergence among fossil and extant amphibians are widespread. Therefore, the assessment of their relationships cannot rely upon comparisons between very few representatives of Palaeozoic and Recent groups or upon selection of a limited number of putative shared derived similarities. Instead, efforts should be directed towardsthe recognition of the group in which the internal relationships best reflect the most coherent, inter-nested set of lissamphibian synapomorphies. We argue that temnospondyls show a coherent nested set of this type.

Crown-tetrapod origin and the apex of the tetrapod stem-group

The following analyses were considered: Carroll 1995 (Figure 11.2); Coates 1996 (Figure11.3); Ahlberg and Clack 1998 (Figure 11.4; see also Figure 11.11 in Appendix 11.1);Laurin and Reisz 1999 (Figure 11.5); Paton et al. 1999 (Figure 11.6); Anderson 2001(Figure 11.7). For each analysis, the inferred minimum age for the lissamphibian-amniotephylogenetic separation is bracketed between 325 and 345 Ma (mid- to late Viséan),in agreement with the conclusions of several previous works (e.g. Clack 1998b,d, 2001,2002; Paton et al. 1999; see also comments in Coates et al. 2000). Importantly, diver-gence time estimates are not affected by the relative positions of unstable/roguetaxa (e.g. baphetids, Caerorhachis, Crassigyrinus, Eucritta, Whatcheeria, and variouslepospondyl groups) or by the degree of tree balance. For example, comparisons betweenLaurin and Reisz’s (1999) analysis (Figure 11.5) and ours (Figure 11.8) reveal a decreasein stem-tetrapod groups, a decrease in stem-lissamphibian groups, and an increase instem-amniote groups. Both analyses, however, place aïstopods within the tetrapodcrown-group (as stem-lissamphibians or stem-amniotes, respectively). These findingsnecessarily imply a mid-Viséan age as a minimum hypothesis for the date of thelissamphibian-amniote separation (Figure 11.9). This is largely based on the mid-Viséanoccurrence of the earliest known aïstopod, Lethiscus (Wellstead 1982).

Stratigraphical data can provide no more than the best approximation of thelissamphibian–amniote divergence time, based on the available sample of fossil mater-ial. The absence of an adequate Tournaisian tetrapod record (Coates and Clack 1995;Lebedev and Coates 1995; Coates 1996; Clack and Finney 1997; Paton et al. 1999;Clack and Carroll 2000; Clack 2002), relative to that from flanking stages, affects

TTEC11 05/06/2003 11:47 AM Page 239


Eu

sth

eno

pte

ron

fo

ord

iP

and

eric

hth

ys r

ho

mb

ole

pis

Aca

nth

ost

ega

gu

nn

ari

Ich

thyo

steg

a st

ensi

oei

Cae

rorh

ach

is b

aird

i

EM

BO

LO

ME

RI

GE

PH

YR

OS

TE

GID

AE

SE

YM

OU

RIA

MO

RP

HA

AIS

TOP

OD

A

AD

EL

OS

PO

ND

YL

I

DIA

DE

CTO

MO

RP

HA

cro

wn

-gro

up

AM

NIO

TA

So

len

od

on

sau

rus

jan

ensc

hi

BA

PH

ET

IDA

E

TE

MN

OS

PO

ND

YL

I

Cra

ssig

yrin

us

sco

ticu

sW

hat

chee

ria

del

tae

CO

LO

ST

EID

AE MIC

RO

SA

UR

IA

NE

CT

RID

EA

LYS

OR

OP

HIA

cro

wn

-gro

up

LIS

SA

MP

HIB

IA

Wes

tlo

thia

na

lizzi

ae


Lochkovian

Pragian

Emsian

Eifelian

Givetian

Frasnian

Famennian

Tournaisian

Visean

Serpukhovian

Bashkirian

Moscovian

Kazimovian

Gzhelian

Asselian

Sakmarian

Artinskian

Kungurian

Roadian

Wordian

Capitanian

Wuchiapigian

Changhsingian

Induan

Olenekian

Anisian

Ladinian

Carnian

Norian

Rhetian

Hettangian

Sinemurian

Pliensbachian

Toarcian

Aalenian

Bajocian

Bathonian

Callovian

Oxfordian

Kimmeridgian

Tithonian

Berriasian

Valanginian

Hauterivian

Barremian

Aptian

Albian

Cenomanian

Turonian

Coniacian

Santonian

Campanian

Maastrichtian

Danian

Thanetian

Ypresian

Lutetian

Barthonian

Priabonian

Rupelian

Chattian

Aquitanian

Burdigalian

Langhian

Serravallian

Tortonian

Messinian

Zanclean

Piacenzian

DE

VO

NIA

NC

AR

BO

NIF

ER

OU

SP

ER

MIA

NT

RIA

SS

ICJU

RA

SS

ICC

RE

TAC

EO

US

EARLY

MIDDLE

LATE

CISURALIAN

PAL

EO

GE

NE

NE

OG

EN

E

GUADALUPIAN

LOPINGIAN

PENNSYLVANIAN

MISSISSIPPIAN

EARLY

MIDDLE

LATE

LATE

MIDDLE

EARLY

EARLY

LATE

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENE

Gelasian

410

400390380375370355

345

325

295

250

240233230220

20320019118417517016416015414614113513112311711310896928887837265

5346403733.72823.520.315.814.3117.35.33.4

1.75

Figure 11.5 Laurin and Reisz’s (1999) analysis plotted on a timescale.

TTEC11 05/06/2003 11:47 AM Page 240


CO

LO

ST

EID

AE

Aca

nth

ost

ega

gu

nn

ari

BA

PH

ET

IDA

E

Cra

ssig

yrin

us

sco

ticu

s

Cae

rorh

ach

is b

aird

i

EM

BO

LO

ME

RI

GE

PH

YR

OS

TE

GID

AE

MIC

RO

SA

UR

IA

SE

YM

OU

RIA

MO

RP

HA

TE

MN

OS

PO

ND

YL

I

Wh

atch

eeri

a d

elta

eE

ucr

itta

mel

ano

limn

etes

cro

wn

-gro

up

AM

NIO

TA

Wes

tlo

thia

na

lizzi

aeC

asin

eria

kid

di


Lochkovian

Pragian

Emsian

Eifelian

Givetian

Frasnian

Famennian

Tournaisian

Visean

Serpukhovian

Bashkirian

Moscovian

Kazimovian

Gzhelian

Asselian

Sakmarian

Artinskian

Kungurian

Roadian

Wordian

Capitanian

Wuchiapigian

Changhsingian

Induan

Olenekian

Anisian

Ladinian

Carnian

Norian

Rhetian

Hettangian

Sinemurian

Pliensbachian

Toarcian

Aalenian

Bajocian

Bathonian

Callovian

Oxfordian

Kimmeridgian

Tithonian

Berriasian

Valanginian

Hauterivian

Barremian

Aptian

Albian

Cenomanian

Turonian

Coniacian

Santonian

Campanian

Maastrichtian

Danian

Thanetian

Ypresian

Lutetian

Barthonian

Priabonian

Rupelian

Chattian

Aquitanian

Burdigalian

Langhian

Serravallian

Tortonian

Messinian

Zanclean

Piacenzian

DE

VO

NIA

NC

AR

BO

NIF

ER

OU

SP

ER

MIA

NT

RIA

SS

ICJU

RA

SS

ICC

RE

TAC

EO

US

EARLY

MIDDLE

LATE

CISURALIAN

PAL

EO

GE

NE

NE

OG

EN

E

GUADALUPIAN

LOPINGIAN

PENNSYLVANIAN

MISSISSIPPIAN

EARLY

MIDDLE

LATE

LATE

MIDDLE

EARLY

EARLY

LATE

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENE

Gelasian

410

400390380375370355

345

325

295

250

240233230220

20320019118417517016416015414614113513112311711310896928887837265

5346403733.72823.520.315.814.3117.3

5.3

3.4

1.75

Figure 11.6 Paton et al.’s (1999) analysis plotted on a timescale.

TTEC11 05/06/2003 11:47 AM Page 241


Aca

nth

ost

ega

gu

nn

ari

NE

CT

RID

EA

AIS

TOP

OD

A

DIA

DE

CTO

MO

RP

HA

EM

BO

LO

ME

RI

SE

YM

OU

RIA

MO

RP

HA

MIC

RO

SA

UR

IA

LYS

OR

OP

HIA

AD

EL

OS

PO

ND

YL

I

TE

MN

OS

PO

ND

YL

I

cro

wn

-gro

up

LIS

SA

MP

HIB

IA

CO

LO

ST

EID

AE


Lochkovian

Pragian

Emsian

Eifelian

Givetian

Frasnian

Famennian

Tournaisian

Visean

Serpukhovian

Bashkirian

Moscovian

Kazimovian

Gzhelian

Asselian

Sakmarian

Artinskian

Kungurian

Roadian

Wordian

Capitanian

Wuchiapigian

Changhsingian

Induan

Olenekian

Anisian

Ladinian

Carnian

Norian

Rhetian

Hettangian

Sinemurian

Pliensbachian

Toarcian

Aalenian

Bajocian

Bathonian

Callovian

Oxfordian

Kimmeridgian

Tithonian

Berriasian

Valanginian

Hauterivian

Barremian

Aptian

Albian

Cenomanian

Turonian

Coniacian

Santonian

Campanian

Maastrichtian

Danian

Thanetian

Ypresian

Lutetian

Barthonian

Priabonian

Rupelian

Chattian

Aquitanian

Burdigalian

Langhian

Serravallian

Tortonian

Messinian

Zanclean

Piacenzian

DE

VO

NIA

NC

AR

BO

NIF

ER

OU

SP

ER

MIA

NT

RIA

SS

ICJU

RA

SS

ICC

RE

TAC

EO

US

EARLY

MIDDLE

LATE

CISURALIAN

PAL

EO

GE

NE

NE

OG

EN

E

GUADALUPIAN

LOPINGIAN

PENNSYLVANIAN

MISSISSIPPIAN

EARLY

MIDDLE

LATE

LATE

MIDDLE

EARLY

EARLY

LATE

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENE

Gelasian

410

400390380375370355

345

325

295

250

240233230220

20320019118417517016416015414614113513112311711310896928887837265

5346403733.72823.520.315.814.3117.35.33.4

1.75

Figure 11.7 Anderson’s (2001) analysis plotted on a timescale.

TTEC11 05/06/2003 11:47 AM Page 242


NE

CT

RID

EA

AIS

TOP

OD

A

DIA

DE

CTO

MO

RP

HA

cro

wn

-gro

up

AM

NIO

TA

So

len

od

on

sau

rus

jan

ensc

hi

Wes

tlo

thia

na

lizzi

ae

Cae

rorh

ach

is b

aird

i

EM

BO

LO

ME

RI

GE

PH

YR

OS

TE

GID

AE

SE

YM

OU

RIA

MO

RP

HA

MIC

RO

SA

UR

IA

LYS

OR

OP

HIA

AD

EL

OS

PO

ND

YL

I

BA

PH

ET

IDA

E

Eu

crit

ta m

elan

olim

net

es

TE

MN

OS

PO

ND

YL

Icr

ow

n-g

rou

pL

ISS

AM

PH

IBIA

Aca

nth

ost

ega

gu

nn

ari

Ich

thyo

steg

a st

ensi

oei

Eu

sth

eno

pte

ron

fo

ord

iP

and

eric

hth

ys r

ho

mb

ole

pis

Ven

tast

ega

curo

nic

a

Tule

rpet

on

cu

rtu

m

Cra

ssig

yrin

us

sco

ticu

sW

hat

chee

ria

del

tae

CO

LO

ST

EID

AE


Lochkovian

Pragian

Emsian

Eifelian

Givetian

Frasnian

Famennian

Tournaisian

Visean

Serpukhovian

Bashkirian

Moscovian

Kazimovian

Gzhelian

Asselian

Sakmarian

Artinskian

Kungurian

Roadian

Wordian

Capitanian

Wuchiapigian

Changhsingian

Induan

Olenekian

Anisian

Ladinian

Carnian

Norian

Rhetian

Hettangian

Sinemurian

Pliensbachian

Toarcian

Aalenian

Bajocian

Bathonian

Callovian

Oxfordian

Kimmeridgian

Tithonian

Berriasian

Valanginian

Hauterivian

Barremian

Aptian

Albian

Cenomanian

Turonian

Coniacian

Santonian

Campanian

Maastrichtian

Danian

Thanetian

Ypresian

Lutetian

Barthonian

Priabonian

Rupelian

Chattian

Aquitanian

Burdigalian

Langhian

Serravallian

Tortonian

Messinian

Zanclean

Piacenzian

DE

VO

NIA

NC

AR

BO

NIF

ER

OU

SP

ER

MIA

NT

RIA

SS

ICJU

RA

SS

ICC

RE

TAC

EO

US

EARLY

MIDDLE

LATE

CISURALIAN

PAL

EO

GE

NE

NE

OG

EN

E

GUADALUPIAN

LOPINGIAN

PENNSYLVANIAN

MISSISSIPPIAN

EARLY

MIDDLE

LATE

LATE

MIDDLE

EARLY

EARLY

LATE

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENE

Gelasian

410

400390380375370355

345

325

295

250

240233230220

20320019118417517016416015414614113513112311711310896928887837265

5346403733.72823.520.315.814.3117.35.33.4

1.75

Figure 11.8 New analysis plotted on a timescale (Ruta et al. in press).

TTEC11 05/06/2003 11:47 AM Page 243


Dip

loca

ulu

s m

agn

ico

rnis

Dip

loce

rasp

is b

urk

ei

Mic

rob

rach

is p

elik

ani

Hyl

op

lesi

on

lon

gic

ost

atu

mO

do

nte

rpet

on

tri

ang

ula

re

Eu

sth

eno

pte

ron

fo

ord

iP

and

eric

hth

ys r

ho

mb

ole

pis

Ven

tast

ega

curo

nic

aA

can

tho

steg

a g

un

nar

iIc

hth

yost

ega

sten

sio

eiTu

lerp

eto

n c

urt

um

Co

lost

eus

scu

tella

tus

Gre

erer

pet

on

bu

rkem

ora

ni

Cra

ssig

yrin

us

sco

ticu

sW

hat

chee

ria

del

tae

Bap

het

es k

irkb

yiM

egal

oce

ph

alu

s p

ach

ycep

hal

us

Eu

crit

ta m

elan

olim

net

esE

do

ps

crai

gi

Co

chle

osa

uru

s fl

ore

nsi

sC

hen

op

roso

pu

s le

wis

iB

alan

erp

eto

n w

oo

di

Iso

dec

tes

ob

tusu

sN

eld

asau

rus

wri

gh

tae

Trim

ero

rhac

his

insi

gn

isD

end

rerp

eto

n a

cad

ian

um

Ery

op

s m

egac

eph

alu

sA

chel

om

a cu

mm

insi

Ph

on

erp

eto

n p

rice

iE

cols

on

ia c

utl

eren

sis

Bro

iliel

lus

bre

vis

Pla

tyrh

ino

ps

lyel

liE

osc

op

us

lock

ard

iA

mp

hib

amu

s g

ran

dic

eps

Do

lese

rpet

on

an

nec

ten

sM

icro

mel

erp

eto

n c

red

ner

iA

pat

eon

ped

estr

isL

epto

rop

hu

s te

ner

Sch

oen

feld

erp

eto

n p

resc

her

iA

lban

erp

eto

n in

exp

ecta

tum

Tria

do

bat

rach

us

mas

sin

oti

Kar

auru

s sh

aro

vi Val

do

trit

on

gra

cilis

Eo

caec

ilia

mic

rop

od

a

Cae

rorh

ach

is b

aird

iA

nth

raco

sau

rus

russ

elli

Arc

her

ia c

rass

idis

caP

ho

lider

pet

on

att

hey

iP

ho

lider

pet

on

scu

tig

eru

mP

rote

rog

yrin

us

sch

eele

iE

oh

erp

eto

n w

atso

ni

Bru

kter

erp

eto

n f

ieb

igi

Gep

hyr

ost

egu

s b

oh

emic

us

So

len

od

on

sau

rus

jan

ensc

hi

Ko

tlas

sia

pri

ma

Dis

cosa

uri

scu

s au

stri

acu

sS

eym

ou

ria

bay

lore

nsi

s / s

anju

anen

sis

Dia

dec

tes

absi

tus

Lim

no

scel

is p

alu

dis

Cap

torh

inu

s ag

uti

Pal

eoth

yris

aca

dia

na

Pet

rola

cosa

uru

s ka

nse

nsi

sW

estl

oth

ian

a liz

ziae

Bat

rop

etes

fri

tsch

iaTu

dit

anu

s p

un

ctu

latu

sP

anty

lus

cho

rdat

us

Ste

go

tret

us

agyr

us

Asa

ph

este

ra in

term

edia

Sax

on

erp

eto

n g

ein

itzi

Hap

sid

op

arei

on

lep

ton

Mic

raro

ter

eryt

hro

gei

os

Pel

od

oso

tis

elo

ng

atu

mR

hyn

cho

nko

s st

ova

lliC

ard

ioce

ph

alu

s st

ern

ber

gi

Eu

ryo

du

s p

rim

us

Bra

chyd

ecte

s el

on

gat

us

/ new

ber

ryi

Sci

nco

sau

rus

cras

sus

Ker

ater

pet

on

gal

van

iB

atra

chid

erp

eto

n r

etic

ula

tum

Dic

erat

osa

uru

s b

revi

rost

ris

Ach

ero

nti

scu

s ca

led

on

iae

Ad

elo

spo

nd

ylu

s w

atso

ni

Ad

elo

gyr

inu

s si

mo

rhyn

chu

sD

olic

ho

par

eias

dis

ject

us

Pty

on

ius

mar

shii

Sau

rop

leu

ra p

ecti

nat

a / s

cala

ris

Uro

cord

ylu

s w

and

esfo

rdii

Let

his

cus

sto

cki

Oes

toce

ph

alu

s am

ph

ium

inu

mP

hle

get

ho

nti

a lin

eari

s

Lochkovian

Pragian

Emsian

Eifelian

Givetian

Frasnian

Famennian

Tournaisian

Visean

Serpukhovian

Bashkirian

Moscovian

Kazimovian

Gzhelian

Asselian

Sakmarian

Artinskian

Kungurian

Roadian

Wordian

Capitanian

Wuchiapigian

Changhsingian

Induan

Olenekian

Anisian

Ladinian

Carnian

Norian

Rhetian

Hettangian

Sinemurian

Pliensbachian

Toarcian

Aalenian

Bajocian

Bathonian

Callovian

Oxfordian

Kimmeridgian

Tithonian

Berriasian

Valanginian

Hauterivian

Barremian

Aptian

Albian

Cenomanian

Turonian

Coniacian

Santonian

Campanian

Maastrichtian

Danian

Thanetian

Ypresian

Lutetian

Barthonian

Priabonian

Rupelian

Chattian

Aquitanian

Burdigalian

Langhian

Serravallian

Tortonian

Messinian

Zanclean

Piacenzian

DE

VO

NIA

NC

AR

BO

NIF

ER

OU

SP

ER

MIA

NT

RIA

SS

ICJU

RA

SS

ICC

RE

TAC

EO

US

EARLY

MIDDLE

LATE

CISURALIAN

PAL

EO

GE

NE

NE

OG

EN

E

GUADALUPIAN

LOPINGIAN

PENNSYLVANIAN

MISSISSIPPIAN

EARLY

MIDDLE

LATE

LATE

MIDDLE

EARLY

EARLY

LATE

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENE

Gelasian

410

400390380375370355

345

325

295

250

240233230220

20320019118417517016416015414614113513112311711310896928887837265

5346403733.72823.520.315.814.3117.35.33.4

1.75

Figure 11.9 Stratigraphical plot of strict consensus of 60 most parsimonious trees at 1303 steps(CI = 0.2357; RI = 0.6744; RC = 0.1641) derived from analysis of cranial and postcra-nial characters and resolved down to species level (Ruta et al. in press).

theories of divergence times, insofar as cladogenetic events can only be plotted withinthe Viséan and later, or within the Famennian and earlier periods. Therefore, the appar-ent consensus between widely conflicting tree topologies about the time of origin ofthe Tetrapoda is only significant because all recent analyses fail to place this eventbefore the Devonian–Carboniferous boundary. However, the observed ‘time signal’

TTEC11 05/06/2003 11:47 AM Page 244


is not exclusively under stratigraphical control, since alternative phylogenies basedon novel character and/or taxon combinations could move the crown-tetrapod originevent to either side of the Tournasian gap. The quality of the signal is neverthelesscompromised by the patchiness of the contributing data. Consequently, characterdeletion experiments (see Ruta et al., in press, and discussion above) are likely to have only minimal effects. Thus, while postcranial character removal increases thenumber of stem-tetrapod taxa (Figure 11.10), this causes only a small change in theminimum estimate of the crown-group divergence time, from mid- to late Viséan.

Crown-lissamphibian origin

There is general agreement on the taxonomic composition of the basal portion of thelissamphibian crown-group (Báez and Basso 1996; Gao and Shubin 2001). The EarlyTriassic stem-salientian Triadobatrachus massinoti from Madagascar is the earliestundisputed crown-lissamphibian, and predates the basal members of the caudate andcaecilian orders – Karaurus sharovi and Eocaecilia micropoda from the Late and EarlyJurassic, respectively (Ivakhnenko 1978; Milner 1988, 1993, 2000; Rage and Rocek1989; Jenkins and Walsh 1993; Carroll 2000; Rocek and Rage 2000b). The MiddleJurassic karaurid caudate Kokartus honorarius (Nessov 1988; Nessov et al. 1996)is older than Karaurus, but is usually regarded as a paedomorphic relative of the latter. Problematic taxa such as Triassurus sixtelae Ivakhnenko, 1978, variouslyinterpreted as a Triassic stem-caudate or as a temnospondyl larva (review in Milner2000), are too poorly known. Likewise, the systematic affinities of various Jurassic‘salamander-like’ taxa (e.g. batrachosauroidids and scapherpetontids) remain uncer-tain (Milner 2000).

The total analysis implies the existence of a mid-Pennsylvanian to Early Triassicghost lineage connecting albanerpetontids plus crown-lissamphibians with a dis-sorophoid assemblage consisting of the amphibamid, micromelerpetontid, and bran-chiosaurid families. The duration of this lineage is disconcertingly longer than thatpostulated by previous studies (Permian to Early Triassic; e.g. Trueb and Cloutier 1991;Milner 1993) and found also in the cranial analysis, wherein the Early Permian Broiliellusis the immediate sister taxon to albanerpetontids and crown-lissamphibians (Figure 11.10). Taken at face value, these results suggest the existence of as yet unknownPermo-Carboniferous taxa into which lissamphibian ancestry is rooted (but see discussion in Ruta et al. in press). No crown-lissamphibian has been recorded in theLate Permian. We point out, however, that our analysis does not consider all knowndissorophoids (reviewed in Milner 1990), most of which require revision. A resolu-tion of the sister group relationships between the three lissamphibian orders and oneor more specific dissorophoid taxa must await a thorough phylogenetic analysis of crownward temnospondyls. Interestingly, Shishkin (1998) discussed a relict andpossibly neotenous dissorophoid (Tungussogyrinus) from the Permian–Triassicboundary in Siberia. However, the affinities of this fossil are uncertain.

Crown-amniote origin

The earliest known, undisputed crown-amniotes date back to the mid-Pennsylvanian(Carroll 1988, 1991; Benton 1991, 1993, 2001; Carroll and Currie 1991; Hopson

TTEC11 05/06/2003 11:47 AM Page 245


Eu

sth

eno

pte

ron

fo

ord

iP

and

eric

hth

ys r

ho

mb

ole

pis

Ven

tast

ega

curo

nic

aA

can

tho

steg

a g

un

nar

i

Ich

thyo

steg

a st

ensi

oei

Tule

rpet

on

cu

rtu

m

Co

lost

eus

scu

tella

tus

Gre

erer

pet

on

bu

rkem

ora

ni

Ker

ater

pet

on

gal

van

i

Ad

elo

spo

nd

ylu

s w

atso

ni

Ad

elo

gyr

inu

s si

mo

rhyn

chu

sD

olic

ho

par

eias

dis

ject

us

Pty

on

ius

mar

shii

Sau

rop

leu

ra p

ecti

nat

a / s

cala

ris

Uro

cord

ylu

s w

and

esfo

rdii

Bra

chyd

ecte

s el

on

gat

us

/ new

ber

ryi

Cra

ssig

yrin

us

sco

ticu

s

Wh

atch

eeri

a d

elta

e

Bap

het

es k

irkb

yiM

egal

oce

ph

alu

s p

ach

ycep

hal

us

Eu

crit

ta m

elan

olim

net

es

Ed

op

s cr

aig

iC

och

leo

sau

rus

flo

ren

sis

Ch

eno

pro

sop

us

lew

isi

Bal

aner

pet

on

wo

od

i

Den

dre

rpet

on

aca

dia

nu

mE

ryo

ps

meg

acep

hal

us

Ach

elo

ma

cum

min

siP

ho

ner

pet

on

pri

cei

Eco

lso

nia

cu

tler

ensi

s

Bro

iliel

lus

bre

vis

Pla

tyrh

ino

ps

lyel

liEo

sco

pu

s lo

ckar

di

Am

ph

ibam

us

gra

nd

icep

sD

ole

serp

eto

n a

nn

ecte

ns

Mic

rom

eler

pet

on

cre

dn

eri

Ap

ateo

n p

edes

tris

Lep

toro

ph

us

ten

erS

cho

enfe

lder

pet

on

pre

sch

eri

Cae

rorh

ach

is b

aird

i

Mic

rob

rach

is p

elik

ani

Hyl

op

lesi

on

lon

gic

ost

atu

m

Bru

kter

erp

eto

n f

ieb

igi

Gep

hyr

ost

egu

s b

oh

emic

us

So

len

od

on

sau

rus

jan

ensc

hi

Ko

tlas

sia

pri

ma

Dis

cosa

uri

scu

s au

stri

acu

sS

eym

ou

ria

bay

lore

nsi

s / s

anju

anen

sis

Dia

dec

tes

absi

tus

Lim

no

scel

is p

alu

dis

Cap

torh

inu

s ag

uti

Pal

eoth

yris

aca

dia

na

Pet

rola

cosa

uru

s ka

nse

nsi

sW

estl

oth

ian

a liz

ziae

Bat

rop

etes

fri

tsch

ia

Tud

itan

us

pu

nct

ula

tus

Asa

ph

este

ra in

term

edia

Sax

on

erp

eto

n g

ein

itzi

Hap

sid

op

arei

on

lep

ton

Pan

tylu

s ch

ord

atu

sS

teg

otr

etu

s ag

yru

s

Mic

raro

ter

eryt

hro

gei

os

Pel

od

oso

tis

elo

ng

atu

mR

hyn

cho

nko

s st

ova

lliC

ard

ioce

ph

alu

s st

ern

ber

gi

Eu

ryo

du

s p

rim

us

Od

on

terp

eto

n t

rian

gu

lare

Alb

aner

pet

on

inex

pec

tatu

m

Tria

do

bat

rach

us

mas

sin

oti

Kar

auru

s sh

aro

viV

ald

otr

ito

n g

raci

lis

Eo

caec

ilia

mic

rop

od

a

Iso

dec

tes

ob

tusu

sN

eld

asau

rus

wri

gh

tae

Trim

ero

rhac

his

insi

gn

is

An

thra

cosa

uru

s ru

ssel

li

Arc

her

ia c

rass

idis

caP

ho

lider

pet

on

att

hey

i

Ph

olid

erp

eto

n s

cuti

ger

um

Pro

tero

gyr

inu

s sc

hee

lei

Eo

her

pet

on

wat

son

i

Oes

toce

ph

alu

s am

ph

ium

inu

m

Ph

leg

eth

on

tia

linea

ris

Let

his

cus

sto

cki

Dip

loca

ulu

s m

agn

ico

rnis

Dip

loce

rasp

is b

urk

ei

Sci

nco

sau

rus

cras

sus

Bat

rach

ider

pet

on

ret

icu

latu

mD

icer

ato

sau

rus

bre

viro

stri

s

Ach

ero

nti

scu

s ca

led

on

iae

Lochkovian

Pragian

Emsian

Eifelian

Givetian

Frasnian

Famennian

Tournaisian

Visean

Serpukhovian

Bashkirian

Moscovian

Kazimovian

Gzhelian

Asselian

Sakmarian

Artinskian

Kungurian

Roadian

Wordian

Capitanian

Wuchiapigian

Changhsingian

Induan

Olenekian

Anisian

Ladinian

Carnian

Norian

Rhetian

Hettangian

Sinemurian

Pliensbachian

Toarcian

Aalenian

Bajocian

Bathonian

Callovian

Oxfordian

Kimmeridgian

Tithonian

Berriasian

Valanginian

Hauterivian

Barremian

Aptian

Albian

Cenomanian

Turonian

Coniacian

Santonian

Campanian

Maastrichtian

Danian

Thanetian

Ypresian

Lutetian

Barthonian

Priabonian

Rupelian

Chattian

Aquitanian

Burdigalian

Langhian

Serravallian

Tortonian

Messinian

Zanclean

Piacenzian

DE

VO

NIA

NC

AR

BO

NIF

ER

OU

SP

ER

MIA

NT

RIA

SS

ICJU

RA

SS

ICC

RE

TAC

EO

US

EARLY

MIDDLE

LATE

CISURALIAN

PAL

EO

GE

NE

NE

OG

EN

E

GUADALUPIAN

LOPINGIAN

PENNSYLVANIAN

MISSISSIPPIAN

EARLY

MIDDLE

LATE

LATE

MIDDLE

EARLY

EARLY

LATE

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENE

Gelasian

410

400390380375370355

345

325

295

250

240233230220

20320019118417517016416015414614113513112311711310896928887837265

5346403733.72823.520.315.814.3117.35.33.4

1.75

Figure 11.10 Stratigraphical plot of strict consensus of 100 440 most parsimonious trees at 945steps (CI = 0.2447; RI = 0.6835; RC = 0.17) derived from analysis of cranial charac-ters and resolved down to species level (Ruta et al. in press).

1991). Both stem-diapsids and primitive synapsids are represented in the Moscovian(families Protorothyrididae and Ophiacodontidae, respectively) and in the Kasimovian(families Petrolacosauridae and Edaphosauridae, respectively). The divergence betweenmammals and sauropsids, placed at about 310 Ma in the Pennsylvanian, has beenwidely used to calibrate molecular clocks (Kumar and Hedges 1998; Hedges 2001;Van Tuinen et al. 2001). The timing of this event is not affected by the controversial

TTEC11 05/06/2003 11:47 AM Page 246


locations of turtles and of various Permo-Carboniferous and Triassic forms in current amniote phylogenies (e.g. Laurin 1991; Laurin and Reisz 1995; Lee 1995, 1997a,b; Rieppel and deBraga 1996; deBraga and Rieppel 1997; Rieppel and Reisz 1999;Rieppel 2000).

The sequence of branching events in the basal part of the amniote stem-group isnot agreed upon. Our analysis reflects established views on the position of such groupsas seymouriamorphs and diadectomorphs, but differs from many previous analysesin the relatively basal position of Solenodonsaurus (Lee and Spencer 1997; Laurinand Reisz 1999). Virtually no fossils have been proposed as immediate sister groupsto amniotes, crownward of diadectomorphs (Sumida and Lombard 1991; Bermanet al. 1992; Lombard and Sumida 1992; Sumida et al. 1992; Laurin and Reisz 1997,1999; Lee and Spencer 1997; Sumida 1997; Berman et al. 1998; Berman 2000, Laurin1998a–c; Ruta et al., in press).

Dating phylogenetic events

Comparison between morphological and molecular analyses

According to Kumar and Hedges (1998) and Hedges (2001), lissamphibians andamniotes diverged at around 360 Ma ± 14.7 myr in the Famennian (Late Devonian;see also Panchen and Smithson 1987, 1988; Lebedev and Coates 1995; Coates 1996).The upper boundary of this time interval falls within the upper part of the Tournaisian,whereas the lower boundary coincides with the basal part of the Frasnian. In all cases,the lissamphibian–amniote divergence is postulated to have occurred earlier than avail-able fossil evidence suggests. The mismatch between molecular and morphological data cannot be explained easily (see Smith 1999, for a comparable example involv-ing metazoan divergence dates, and other chapters in this volume). However, we notethat the mid- to late Viséan separation between lissamphibians and amniotes inferredfrom morphological analyses falls slightly short of the upper boundary of Kumar andHedges’s (1998) and Hedges’ (2001) time interval. As mentioned above, some tetrapodhumeri from Tournaisian sediments in Nova Scotia (Clack and Carroll 2000) resemblein their general proportions those of certain Viséan ‘reptiliomorphs’, notablyEoherpeton (Smithson 1985), although the evidence is not compelling. Furthermore,the same sediments have yielded putative colosteid-like humeri, suggesting the occur-rence of deeper branching events for at least some tetrapod groups. In addition, thepresence of the stem-amniote Casineria in the middle part of the late Viséan showsthat ‘reptiliomorph’ diversification was already under way by about 340 Ma (Patonet al. 1999).

It is much more difficult to reconcile a Frasnian age for the lissamphibian–amniotephylogenetic split (the lower boundary of Kumar and Hedges’ 1998, and Hedge’s 2001,time interval) with available fossil data (summary in Coates, 2001). All Devoniantetrapods with limbs postdate fish-like stem-tetrapods, such as Panderichthys andEusthenopteron. None of them are currently regarded as a member of the crown-group.Also, the basal node of the tetrapod crown-group cannot be rooted into known Devoniantaxa (e.g. Panchen and Smithson 1988) without implying an impressive series ofconvergent character-changes in the most basal portions of the lissamphibian andamniote stem-groups. However, putative tetrapod trackways recorded in Australia,

TTEC11 05/06/2003 11:47 AM 7


Ireland, and Scotland (see reviews in Clack 1998a, 2000) could be used to hypothe-size the existence of an as yet unrecorded radiation of limbed tetrapods during theFrasnian–Famennian. The dating of several track-bearing sediments is disputed, butin certain cases, a Middle to Late Devonian age has been postulated.

A better agreement between morphological and molecular time-calibrated trees isevident by comparing minimum estimates of crown-lissamphibian origins (see alsoabove). Thus, both morphological analyses and molecular studies (e.g. Báez and Basso1996; Feller and Hedges 1998; Gao and Shubin 2001) support an early Mesozoic divergence for crown-lissamphibians. According to Feller and Hedges (1998), the EarlyTriassic age of Triadobatrachus implies that the three orders of lissamphibiansoriginated in the Palaeozoic under the traditional hypothesis of a sister grouprelationship between salientians and caudates. Indeed, the morphology of Triado-batrachus appears almost exactly intermediate between that of more derived frogs andvarious derived dissorophoids (Milner, 1988; Rocek and Rage 2000a,b). Althoughthe gymnophionan–caudate clade [= Procera] proposed by Feller and Hedges (1998)may imply a later evolutionary event for the origin of caecilians and salamandersrelative to frogs, this branching sequence is not incompatible with the possibility thatpre-Jurassic (or even Late Palaeozoic) representatives of caecilians and salamandersmay be discovered. Although Feller and Hedges (1998) found morphological supportfor their Procera, it is at present difficult to propose a suitable candidate for thestem-group membership of this clade (but see McGowan and Evans 1995).

Conflict or compromise?

Agreement between morphology and molecules in reconstructing the timing of majorevolutionary events is rare. Discrepancies between different data sources for severaltaxonomic groups are well documented. In the case of metazoans, birds, andmammals, for instance, molecular analyses indicate that these groups are twice as oldas their oldest fossil representatives. Instances of molecular estimates falling shortof morphological estimates exist, but are much rarer (e.g. Easteal and Herbert 1997).Several factors have been identified as responsible for the mismatch betweenmolecules and morphology (cf. Cooper and Fortey 1998; Benton 1999; Smith 1999),including the presumed rarity of ancestral forms of major groups (let alone problemswith the recognition of ancestors), their preservation potential, and their possibleoccurrence in places that have not yet been subject to thorough scrutiny. Further-more, failure to distinguish between the origin of the living members of a Recent clade(crown-group diversification) and the date of separation of the latter from its extantsister group (total-group divergence) may lead to biased assessments of originationtimes (e.g. Easteal 1999). For instance, assuming the accuracy of our new hypothesisof tetrapod relationships (Ruta et al. in press), a time interval of about 30 millionyears separates the earliest undisputed crown-amniotes from Casineria. On the lis-samphibian stem, the time interval between the earliest undisputed crown-lissamphibian,Triadobatrachus, and the earliest known temnospondyls is about 75 million years(Figure 11.9). Furthermore, as pointed out by van Tuinen et al. (2001), fossil-basedcalibrations of molecular clocks are inevitably sensitive to fossil dating and phylogenyreconstruction (for a comprehensive discussion, see also Wagner 2000). For thisreason, they emphasize the importance of introducing confidence limits around such

TTEC11 05/06/2003 11:47 AM Page 248


widely used, fossil-based calibration tools as the synapsid–diapsid divergence time (see above).

Another important issue is represented by the erratic behaviour of molecularclocks, a discussion of which was presented by Ayala (1999). Briefly, several factors(e.g. population size, time elapsed between generations, species-specific occurrencesof genetic mutations, changes in protein functions, and changes in the adaptation oforganisms to their environments) may speed up or slow down molecular clocks (Cooperand Fortey 1998; Benton 1999; Smith 1999). Examination of combined informationfrom a large number of genes has been proposed as an effective tool to reduce drast-ically the errors introduced by limited sequence data (e.g. Kumar and Hedges 1998;Ayala 1999; Hedges 2001; Stauffer et al. 2001). The discussion thus far shows thatthe most problematic incongruence between molecular and morphological time treesconcerns the age of the tetrapod crown-group radiation. This lack of agreement couldresult from inaccuracies of molecular clock estimates. Smith (1999) has summarizedcases in which rates of molecular evolution might change dramatically, both at thestart of clade radiation, and in terminal portions of the tree relative to deeper nodes.For example, if genetic changes in a sufficient number of gene families were sloweddown at the beginning of the crown-amniote radiation (one of the most widely usedcalibration points; Feller and Hedges 1998; Kumar and Hedges 1998; Hedges 2001),then molecular data would deliver an excessively early origination date; certainly mucholder than that estimated from fossils. We note that such a model of varying molecu-lar clock-speed is consistent with the greater agreement between molecular and morpho-logical estimates of crown-lissamphibian origin (since the crown-lissamphibian radiation is far more recent than that of crown-tetrapods).

Sample bias and ‘site’ effect

Improved molecular methods and techniques (e.g. Hedges 2001), and increasedconsistency of divergence times, between different gene samples and calibrationpoints (e.g. Stauffer et al. 2001), make it appear a priori that the mismatch betweenpalaeontological and molecular estimates for divergence times is caused by deficien-cies of the fossil record. However, this is strongly disputed in the case of certain groups(notably, birds and mammals; Benton 1999). Sample bias is an important factor whendealing with palaeontological data. Benton and Hitchin (1996) and Benton et al. (2000)used cladograms from a wide range of groups to test the quality of the fossil record,which they acknowledge as decreasing dramatically backwards in time. Older fossilsare more liable to physical and chemical destruction than younger ones. The formerare often more difficult to interpret and to place in a phylogenetic context than thelatter. In addition, it is reasonable to assume that taxa that lie phylogenetically closeto cladogenetic events are rare.

Benton and Hitchin (1996) and Benton et al. (2000) argue that, although the‘completeness’ of the fossil record may be lower in the Palaeozoic than in theCenozoic, its ‘adequacy’ in recounting major evolutionary events is maintained.Newly discovered taxa are more likely to fit within well-established higher categories,and to redefine only lower ranks (e.g. splitting or clumping genera and species). Itfollows that differences in fossil dating are only significant at the level of finechronostratigraphical subdivisions (e.g. stages). The quality of the fossil record is

TTEC11 05/06/2003 11:47 AM Page 249


thus interpreted as more or less uniform when families are used as OTUs and the strati-graphical column is scaled to stages. Therefore, it is unlikely that discoveries of newmembers of well-characterized Palaeozoic tetrapod clades will have any impact uponthe branching sequence and chronology of key events in tetrapod history (althoughthey may cast new light on the intrinsic relationships of the groups to which they belong).Nevertheless, certain discoveries are crucial, as in the case of fossils displayingmosaics of features previously considered diagnostic of higher level, distinct clades(Clack 2001; Ruta et al. 2001). Moreover, the methodology employed by Bentonand Hitchin (1996) and Benton et al. (2000) treats phylogenetic reconstruction asindependent of sampling order, even though sampling intensity (the probability thata taxon is sampled per given unit time) affects phylogenetic accuracy (Wagner 2000,and references therein).

Large gaps in the early tetrapod record, most notably the Tournaisian, persist. Samplequality from this time interval is thus extremely poor compared with more recent deposits(Benton 1999). In fact, most discoveries of early tetrapods have resulted from pro-longed, concentrated collecting efforts in a limited number of stratigraphical horizons(e.g. Wood et al. 1985; Rolfe et al. 1994), although fortuitous finds remain anoccasional source of important new data (Clack and Finney 1997; Paton et al. 1999).Consequently, certain key fossil sites have a disproportionate influence, most partic-ularly East Kirkton in the Scottish late Viséan (Rolfe et al. 1994). Key East Kirktontaxa responsible for pegging divergence dates on the tree include the putativestem-lissamphibians Balanerpeton and Eucritta, and the stem-amniote Westlothiana(Figure 11.1). There is nothing unique to East Kirkton and early tetrapod phylogenyin this respect; such site effects are applicable to the vast majority of fossil-basedestimates of evolutionary timing.

Phylogenetic reconstructions cannot be regarded as finished works, because thediscovery of just one new fossil may overturn previous hypotheses about characterdistribution and polarity. Therefore, fossil-based estimates of major evolutionary eventsare not necessarily in conflict with, or challenged by, existing molecular estimates.However, we point out that this is true only if molecular estimates exceed those impliedby morphology. If fossil estimates exceed molecular estimates, then it appears to usthat a real conflict exists. As suggested by Stauffer et al. (2001), one of the best usesfor molecular clock time trees is their ability to provide a framework to evaluate (and,possibly, constrain) palaeontological hypotheses of divergence.Therefore, in agreementwith Hedges and Maxson (1997), molecular and palaeontological data are best usedas complementary approaches to dating phylogenetic events.

Acknowledgements

We thank Drs Philip Donoghue and Paul Smith (University of Birmingham, UK) forinviting us to contribute this paper to the one-day symposium ‘Telling the evolutionarytime: molecular clocks and the fossil record’ at the Third Biennial Meeting of theSystematics Association, Imperial College, University of London, UK. We aregrateful to them and to Dr Per E. Ahlberg (NHM, London, UK) for their editorialcomments, stylistic suggestions, and constructive criticism of an earlier draft of thiswork. We benefited from exchange of ideas with Prof. S. Blair Hedges (PennsylvaniaState University, USA). Marcello Ruta acknowledges the financial support provided

TTEC11 05/06/2003 11:47 AM Page 250


by the Palaeontological Association. This work is part of a research project fundedby BBSRC Advanced Research Fellowship no. 31/AF/13042 awarded to Michael I.Coates.

References

Ahlberg. P.E. and Clack, J.A. (1998) ‘Lower jaws, lower tetrapods – a review based on theDevonian genus Acanthostega’, Transactions of the Royal Society of Edinburgh: EarthSciences, 89: 11–46.

Ahlberg, P.E. and Johanson, Z. (1998) ‘Osteolepiforms and the ancestry of tetrapods’, Nature,395: 792–4.

Ahlberg, P.E. and Milner, A.R. (1994) ‘The origin and early diversification of tetrapods’, Nature,368: 507–14.

Anderson, J.S. (2001) ‘The phylogenetic trunk: maximal inclusion of taxa with missing datain an analysis of the Lepospondyli (Vertebrata, Tetrapoda)’, Systematic Biology, 50:170–93.

Ayala, F.J. (1999) ‘Molecular clock mirages’, BioEssays, 21: 71–5.Báez, A.M. and Basso, N.G. (1996) ‘The earliest known frogs of the Jurassic of South America:

review and cladistic appraisal of their relationships’, Münchner GeowissenschaftlicheAbhandlungen, Reihe A (Geologie und Paläontologie), 30: 131–58.

Beaumont, E.H. (1977) ‘Cranial morphology of the Loxommatidae (Amphibia:Labyrinthodontia)’, Philosophical Transactions of the Royal Society, London, B280:29–101.

Beaumont, E.H. and Smithson, T.R. (1998) ‘The cranial morphology and relationships of theaberrant Carboniferous amphibian Spathicephalus mirus Watson’, Zoological Journal of theLinnean Society, 122: 187–209.

Benton, M.J. (1991) ‘Amniote phylogeny’, in H.-P. Schultze and L. Trueb (eds) Origins of theHigher Groups of Tetrapods: Controversy and Consensus, Ithaca: Cornell University Press,pp. 317–30.

—— (1993) The Fossil Record 2, London: Chapman & Hall.—— (1999) ‘Early origins of modern birds and mammals: molecules vs. morphology’,

BioEssays, 21: 1043–51.—— (2001) Vertebrate Palaeontology, London: Chapman and Hall.Benton, M.J. and Hitchin, R. (1996) ‘Testing the quality of the fossil record by groups and by

major habitats’, Historical Biology, 12: 111–57.Benton, M.J., Wills, M.A. and Hitchin, R. (2000) ‘Quality of the fossil record through time’,

Nature, 403: 534–7.Berman, D.S. (2000) ‘Origin and early evolution of the amniote occiput’, Journal of

Paleontology, 74: 938–56.Berman, D.S., Sumida S.S. and Lombard, R.E. (1992) ‘Reinterpretation of the temporal and

occipital regions in Diadectes and the relationships of diadectomorphs’, Journal ofPaleontology, 66: 481–99.

Berman, D.S., Sumida, S.S. and Martens, T. (1998) ‘Diadectes (Diadectomorpha: Diadectidae)from the early Permian of central Germany, with description of a new species’, Annals ofthe Carnegie Museum, 67: 53–93.

Bolt, J.R. (1969) ‘Lissamphibian origins: possible protolissamphibian from the Lower Permianof Oklahoma’, Science, 166: 888–91.

—— (1977) ‘Dissorophoid relationships and ontogeny, and the origin of the Lissamphibia’,Journal of Paleontology, 51: 235–49.

—— (1979) ‘Amphibamus grandiceps as a juvenile dissorophid: evidence and implications’, inM.H. Nitecki (ed.) Mazon Creek Fossils, New York: Academic Press, pp. 529–63.

TTEC11 05/06/2003 11:47 AM Page 251


Bolt, J.R. (1991) ‘Lissamphibian origins’, in H.-P. Schultze and L. Trueb (eds) Origins of theHigher Groups of Tetrapods: Controversy and Consensus, Ithaca: Cornell University Press,pp. 194–222.

Bolt, J.R. and Chatterjee, S. (2000) ‘A new temnospondyl amphibian from the late Triassic ofTexas’, Journal of Paleontology, 74: 670–83.

Bolt, J.R. and Lombard, R.E. (1985) ‘Evolution of the tympanic ear and the origin of frogs’,Biological Journal of the Linnean Society, 24: 83–99.

—— (2000) ‘Palaeobiology of Whatcheeria deltae, a primitive Mississippian tetrapod’, inH. Heatwole and R.L. Carroll (eds) Amphibian Biology, 4: Palaeontology, ChippingNorton: Surrey Beatty & Sons, pp. 1044–52.

—— (2001) ‘The mandible of the primitive tetrapod Greererpeton, and the early evolution ofthe tetrapod lower jaw’, Journal of Paleontology, 75: 1016–42.

Bossy, A.K. and Milner, A.C. (1998) ‘Order Nectridea Miall, 1875’, in P. Wellnhofer (ed.)Handbuch der Paläoherpetologie, Teil 1: Lepospondyli, Munich: Pfeil Verlag, pp. 73–131.

Boy, J.A. and Sues, H.-D. (2000) ‘Branchiosaurs: larvae, metamorphosis and heterochrony intemnospondyls and seymouriamorphs’, in H. Heatwole and R.L. Carroll (eds) AmphibianBiology, 4: Palaeontology, Chipping Norton: Surrey Beatty & Sons, pp. 1150–97.

Briggs, D.E.G. and Crowther P.R. (eds) (2001) Palaeobiology II, London: Blackwell Press.Budd, G. (2001) ‘Climbing life’s tree’, Nature, 412: 487.Carroll, R.L. (1988) Vertebrate Paleontology and Evolution, New York: Freeman.—— (1991) ‘The origin of reptiles’, in H.-P. Schultze and L. Trueb (eds) Origins of the Higher

Groups of Tetrapods: Controversy and Consensus, Ithaca: Cornell University Press, pp. 331–53.—— (1995) ‘Problems of the phylogenetic analysis of Paleozoic choanates’, in M. Arsenault,

H. Leliévre and P. Janvier (eds) Studies on Early Vertebrates: (VIIth InternationalSymposium, Parc de Miguasha, Quebec), Bulletin du Museum National d’HistoireNaturelle, Paris (Series 4), 17 (C): 389–445.

—— (1999) ‘Homology among divergent Paleozoic tetrapod clades’, in G.R. Bock and G. Cardew(eds) Homology, Chichester: Wiley, pp. 47–64.

—— (2000) ‘Eocaecilia and the origin of caecilians’, in H. Heatwole and R.L. Carroll (eds)Amphibian Biology, 4: Palaeontology, Chipping Norton: Surrey Beatty & Sons, pp. 1402–11.

—— (2001) ‘The origin and early radiation of terrestrial vertebrates’, Journal of Paleontology,75: 1202–13.

Carroll, R.L. and Bolt, J.R. (2001) ‘The Paleozoic divergence of frogs and salamanders’,Journal of Vertebrate Paleontology, 21 (3 – supplement): 38A.

Carroll, R.L. and Currie, P.J. (1975) ‘Microsaurs as possible apodan ancestors’, ZoologicalJournal of the Linnean Society, 57: 229–47.

—— (1991) ‘The early radiation of diapsid reptiles’, in H.-P. Schultze and L. Trueb (eds) Originsof the Higher Groups of Tetrapods: Controversy and Consensus, Ithaca: Cornell UniversityPress, pp. 354–424.

Carroll, R.L. and Gaskill, P. (1978) ‘The Order Microsauria’, Memoirs of the AmericanPhilosophical Society, 126: 1–211.

Clack, J.A. (1994) ‘Silvanerpeton miripedes, a new anthracosauroid from the Viséan of EastKirkton, West Lothian, Scotland’, Transactions of the Royal Society of Edinburgh: EarthSciences, 84: 369–76.

—— (1996) ‘The palate of Crassigyrinus scoticus, a primitive tetrapod from the LowerCarboniferous of Scotland’, in A.R. Milner (ed.) Studies on Carboniferous and PermianVertebrates, Special Papers in Palaeontology, 52: 55–64.

—— (1998a) ‘Devonian tetrapod trackways and trackmakers; a review of the fossils andfootprints’, Palaeogeography, Palaeoclimatology and Palaeoecology, 130: 227–50.

—— (1998b) ‘A new Early Carboniferous tetrapod with a mèlange of crown-group charac-ters’, Nature, 394: 66–9.

TTEC11 05/06/2003 11:47 AM Page 252


—— (1998c) ‘The neurocranium of Acanthostega gunnari and the evolution of the otic regionin tetrapods’, Zoological Journal of the Linnean Society, 122: 61–97.

—— (1998d) ‘The Scottish Carboniferous tetrapod Crassigyrinus scoticus (Lydekker) – cranialanatomy and relationships’, Transactions of the Royal Society of Edinburgh: Earth Sciences,88: 127–42.

—— (2000) ‘The origin of tetrapods’, in H. Heatwole and R.L. Carroll (eds) AmphibianBiology, 4: Palaeontology, Chipping Norton: Surrey Beatty & Sons, pp. 979–1029.

—— (2001) ‘Eucritta melanolimnetes from the Early Carboniferous of Scotland, a stem tetra-pod showing a mosaic of characteristics’, Transactions of the Royal Society of Edinburgh:Earth Sciences, 92: 75–95.

—— (2002) ‘An early tetrapod from “Romer’s Gap”’, Nature, 418: 72–6.Clack, J.A. and Carroll, R.L. (2000) ‘Early Carboniferous tetrapods’, in H. Heatwole and

R.L. Carroll (eds) Amphibian Biology, 4: Palaeontology, Chipping Norton: Surrey Beatty &Sons, pp. 1030–43.

Clack, J.A. and Finney, S.M. (1997) ‘An articulated tetrapod specimen from the Tournaisianof western Scotland’, Journal of Vertebrate Paleontology, 17 (3 – supplement): 38A.

Cloutier, R. and Ahlberg, P.E. (1996) ‘Morphology, characters, and the inter-relationshipsof basal sarcopterygians’, in M.L.J. Stiassny, L.R. Parenti and G.D. Johnson (eds) Inter-relationships of Fishes, London: Academic Press, pp. 325–37.

Coates, M.I. (1996) ‘The Devonian tetrapod Acanthostega gunnari Jarvik: postcranialanatomy, basal tetrapod inter-relationships and patterns of skeletal evolution’, Transactionsof the Royal Society of Edinburgh: Earth Sciences, 87: 363–421.

—— (2001) ‘Origin of tetrapods’, in D.E.G. Briggs and P.R. Crowther (eds) Palaeobiology II,London: Blackwell Press, pp. 74–9.

Coates, M.I. and Clack, J.A. (1990) ‘Polydactyly in the earliest known tetrapod limbs’,Nature, 347: 66–9.

—— (1991) ‘Fish-like gills and breathing in the earliest known tetrapod’, Nature, 352: 234–6.

—— (1995) ‘Romer’s Gap – tetrapod origins and terrestriality’, in M. Arsenault, H. Lelièvreand P. Janvier (eds) Studies on Early Vertebrates: (VIIth International Symposium, Parc deMiguasha, Quebec), Bulletin du Museum National d’Histoire Naturelle, Paris (Series 4), 17(C): 373–88.

Coates, M.I., Ruta, M. and Milner, A.R. (2000) ‘Early tetrapod evolution’, Trends in Ecologyand Evolution, 15: 327–8.

Cooper, A. and Fortey, R.A. (1998) ‘Evolutionary explosions and the phylogenetic fuse’,Trends in Ecology and Evolution, 13: 151–6.

Craske, A.J. and Jefferies, R.P.S. (1989) ‘A new mitrate from the Upper Ordovician ofNorway, and a new approach to subdividing a plesion’, Palaeontology, 32: 69–99.

Daeschler, E.B., Shubin, N., Thomson, K.S. and Amaral, W.W. (1994) ‘A Devonian tetrapodfrom North America’. Science, 265: 639–42.

deBraga, M. and Rieppel, O. (1997) ‘Reptile phylogeny and the inter-relationships of turtles’,Zoological Journal of the Linnean Society, 120: 281–354.

Duellman, W.E. (1988) ‘Evolutionary relationships of the Amphibia’, in B. Fritzsch, M.J. Ryan,W. Wilczynski, T.E. Hetherington and W. Walkowiak (eds) The Evolution of theAmphibian Auditory System, New York: Wiley, pp. 13–34.

Duellman, W.E. and Trueb, L. (1986) Biology of Amphibians, New York: McGraw-Hill.Easteal, S. (1999) ‘Molecular evidence for the early divergence of placental mammals’,

BioEssays, 21: 1052–8.Easteal, S and Herbert, G. (1997) ‘Molecular evidence from the nuclear genome for the time

frame of human evolution’, Journal of Molecular Evolution, 44: 121–32.Feller, A.E. and Hedges, S.B. (1998) ‘Molecular evidence for the early history of living amphi-

bians’, Molecular Phylogeny and Evolution, 9: 509–16.

TTEC11 05/06/2003 11:47 AM Page 253


Gao, K.-Q. and Shubin, N.H. (2001) ‘Late Jurassic salamanders from northern China’,Nature, 410: 574–7.

Gardner, J.D. (2001) ‘Monophyly and affinities of albanerpetontid amphibians(Temnospondyli; Lissamphibia)’, Zoological Journal of the Linnean Society, 131: 309–52.

Gauthier, J.A., Kluge, A.G. and Rowe, T. (1988a) ‘The early evolution of the Amniota’,in M.J. Benton (ed.) The Phylogeny and Classification of the Tetrapods, 1: Amphibians, Reptiles,Birds, Oxford: Clarendon Press, pp. 103–55.

—— (1988b) ‘Amniote phylogeny and the importance of fossils’, Cladistics, 4: 105–209.Godfrey, S.J. (1989) ‘The postcranial skeletal anatomy of the Carboniferous tetrapod

Greererpeton burkemorani’, Philosophical Transactions of the Royal Society, London,B323: 75–133.

Hay, J.M., Ruvinsky, I., Hedges, S.B. and Maxson, L.R. (1995) ‘Phylogenetic relationships ofamphibian families inferred from DNA sequences of mitochondrial 12S and 16S ribosomalRNA genes’, Molecular Biology and Evolution, 12: 928–37.

Heaton, M.J. (1980) ‘The Cotylosauria: a reconsideration of a group of archaic tetrapods’,in A.L. Panchen (ed.) The Terrestrial Environment and the Origin of Land Vertebrates, London:Academic Press, pp. 497–551.

Hedges, S.B. (2001) ‘Molecular evidence for the early history of living vertebrates’, in P.E. Ahlberg(ed.) Major Events in Early Vertebrate Evolution: Palaeontology, Phylogeny, Genetics andDevelopment, London: Taylor & Francis, pp. 119–34.

Hedges, S.B. and Maxson L.R. (1993) ‘A molecular perspective on Lissamphibian phylogeny’,Herpetological Monographs, 7: 27–42.

—— (1997) ‘Complementary uses of molecules and morphology’, Molecular Phylogenetics andEvolution, 8: 445.

Hedges, S.B. and Poling, L.L. (1999) ‘A molecular phylogeny of reptiles’, Science, 283:998–1001.

Hedges, S.B., Moberg, K.D. and Maxson, L.R. (1990) ‘Tetrapod phylogeny inferred from 18Sand 28S ribosomal RNA sequences and a review of the evidence for amniote phylogeny’,Molecular Biology and Evolution, 7: 607–33.

Hennig, W. (1966) Phylogenetic Systematics, Urbana: University of Illinois Press.Holmes, R.B. (2000) ‘Palaeozoic temnospondyls’, in H. Heatwole and R.L. Carroll (eds)

Amphibian Biology, 4: Palaeontology, Chipping Norton: Surrey Beatty & Sons, pp. 1081–1120.

Holmes, R.B. and Carroll, R.L. (1977) ‘A temnospondyl amphibian from the Mississippian ofScotland’, Bulletin of the Museum of Comparative Zoology, Harvard University, 147: 489–511.

Hook, R.W. (1983) ‘Colosteus scutellatus (Newberry), a primitive temnospondyl amphibian fromthe Middle Pennsylvanian of Linton, Ohio’, American Museum Novitates, 2770: 1–41.

Hopson, J.A. (1991) ‘Systematics of the nonmammalian Synapsida and implications forpatterns of evolution in synapsids’, in H.-P. Schultze and L. Trueb (eds) Origins of the HigherGroups of Tetrapods: Controversy and Consensus, Ithaca: Cornell University Press, pp. 635–93.

Ivakhnenko, M.F. (1978) ‘Urodelans from the Triassic and Jurassic of Soviet Central Asia’,Paleontological Journal, 1978: 84–9.

Janvier, P. (1996) Early Vertebrates, Oxford: Clarendon Press.Jarvik, E. (1996) ‘The Devonian tetrapod Ichthyostega’, Fossils and Strata, 40: 1–206.Jefferies, R.P.S. (1979) ‘The origin of chordates – a methodological essay’, in M.R. House (ed.)

The Origin of Major Invertebrate Groups, Systematics Association Special Volume 12:443–77.

Jeffery, J.E. (2001) ‘Pectoral fins of rhizodontids and the evolution of pectoral appendages inthe tetrapod stem-group’, Biological Journal of the Linnean Society, 74: 217–36.

Jenkins, F.A., Jr. and Walsh, D. (1993) ‘An Early Jurassic caecilian with limbs’, Nature, 365:246–50.

TTEC11 05/06/2003 11:47 AM Page 254


Johanson, Z. and Ahlberg, P.E. (2001) ‘Devonian rhizodontids and tristichopterids(Sarcopterygii; Tetrapodomorpha) from East Gondwana’, Transactions of the Royal Societyof Edinburgh: Earth Sciences, 92: 43–74.

Kearney, M. (2002) ‘Fragmentary taxa, missing data, and ambiguity: mistaken assumptionsand conclusions’, Systematic Biology, 51: 369–81.

Kumar, S. and Hedges, S.B. (1998) ‘A molecular timescale for vertebrate evolution’, Nature,392: 917–20.

Laurin, M. (1991) ‘The osteology of a Lower Permian eosuchian from Texas and a review ofdiapsid phylogeny’, Zoological Journal of the Linnean Society, 101: 59–95.

—— (1998a) ‘The importance of global parsimony and historical bias in understandingtetrapod evolution. Part I. Systematics, middle ear evolution, and jaw suspension’, Annalsdes Sciences Naturelles, Zoologie, 19: 1–42.

—— (1998b) ‘The importance of global parsimony and historical bias in understandingtetrapod evolution. Part II. Vertebral centrum, costal ventilation, and paedomorphosis’, Annalsdes Sciences Naturelles, Zoologie, 19: 99–114.

—— (1998c) ‘A reevaluation of the origin of pentadactyly’, Evolution, 52: 1476–82.—— (2000) ‘Seymouriamorphs’, in H. Heatwole and R.L. Carroll (eds) Amphibian Biology,

4: Palaeontology, Chipping Norton: Surrey Beatty & Sons, pp. 1064–80.Laurin, M. and Reisz, R.R. (1995) ‘A reevaluation of early amniote phylogeny’, Zoological

Journal of the Linnean Society, 113: 165–223.—— (1997) ‘A new perspective on tetrapod phylogeny’, in S.S. Sumida and K.L.M. Martin (eds)

Amniote Origins: Completing the Transition to Land, London: Academic Press, pp. 9–59.—— (1999) ‘A new study of Solenodonsaurus janenschi, and a reconsideration of amniote

origins and stegocephalian evolution’, Canadian Journal of Earth Sciences, 36: 1239–55.Laurin, M., Girondot, M. and de Ricql’s, A. (2000a) ‘Early tetrapod evolution’, Trends in Ecology

and Evolution, 15: 118–23.—— (2000b) ‘Reply’, Trends in Ecology and Evolution, 15: 328.Lebedev, O.A. (1984) ‘First discovery of a Devonian tetrapod vertebrate in USSR’, Doklady

Akademii Nauk SSSR, 278: 1470–3.Lebedev, O.A. and Clack, J.A. (1993) ‘Upper Devonian tetrapods from Andreyevka, Tula region,

Russia’, Palaeontology, 36: 721–34.Lebedev, O.A. and Coates, M.I. (1995) ‘The postcranial skeleton of the Devonian tetrapod

Tulerpeton curtum Lebedev’, Zoological Journal of the Linnean Society, 114: 307–48.Lee, M.S.Y. (1993) ‘The origin of the turtle body plan: bridging a famous morphological gap’,

Science, 261: 1716–20.—— (1995) ‘Historical burden in systematics and the inter-relationships of “Parareptiles”’,

Biological Reviews, 70: 459–547.—— (1996) ‘Correlated progression and the origin of turtles’, Nature, 379: 812–5.—— (1997a) ‘Pareiasaur phylogeny and the origin of turtles’, Zoological Journal of the

Linnean Society, 120: 197–280.—— (1997b) ‘Reptile relationships turn turtle’, Nature, 389: 245.Lee, M.S.Y. and Spencer, P.S. (1997) ‘Crown-clades, key characters and taxonomic stability:

when is an amniote not an amniote?’, in S.S. Sumida and K.L.M. Martin (eds) Amniote Origins:Completing the Transition to Land, London: Academic Press, pp. 61–84.

Lombard, R.E. and Bolt, J.R. (1979) ‘Evolution of the tetrapod ear: an analysis and reinter-pretation’, Biological Journal of the Linnean Society, 11: 19–76.

—— (1995) ‘A new primitive tetrapod Whatcheeria deltae from the Lower Carboniferousof Iowa’, Palaeontology, 38: 471–94.

—— (1999) ‘A microsaur from the Mississippian of Illinois and a standard format formorphological characters’, Journal of Paleontology, 73: 908–23.

Lombard, R.E. and Sumida, S.S. (1992) ‘Recent progress in understanding early tetrapods’,American Zoologist, 32: 609–22.

TTEC11 05/06/2003 11:47 AM Page 255


McGowan, G. and Evans, S.E. (1995) ‘Albanerpetontid amphibians from the Early Cretaceousof Spain’, Nature, 373: 143–5.

Milner, A.C. and Lindsay, W. (1998) ‘Postcranial remains of Baphetes and their bearing onthe relationships of the Baphetidae (= Loxommatidae)’, Zoological Journal of the LinneanSociety, 122: 211–35.

Milner, A.R. (1988) ‘The relationships and origin of living amphibians’, in M.J. Benton (ed.)The Phylogeny and Classification of the Tetrapods, 1: Amphibians, Reptiles, Birds, Oxford:Clarendon Press, pp. 59–102.

—— (1990) ‘The radiations of temnospondyl amphibians’, in P.D. Taylor and G.P. Larwood(eds) Major Evolutionary Radiations, Oxford: Clarendon Press, pp. 321–49.

—— (1993) ‘The Paleozoic relatives of lissamphibians’, Herpetological Monographs, 7: 8–27.

—— (2000) ‘Mesozoic and Tertiary Caudata and Albanerpetontidae’, in H. Heatwole and R.L.Carroll (eds) Amphibian Biology, 4: Palaeontology, Chipping Norton: Surrey Beatty & Sons,pp. 1412–44.

Milner, A.R. and Sequeira, S.E.K. (1994) ‘The temnospondyl amphibians from the Viséan ofEast Kirkton, West Lothian, Scotland’, Transactions of the Royal Society of Edinburgh: EarthSciences, 84: 331–61.

Nessov, L.A. (1988) ‘Late Mesozoic amphibians and lizards of Soviet Middle Asia’, Acta ZoologicaCracoviensia, 31: 475–86.

Nessov, L.A., Fedorov, P.V., Potanov, D.O. and Golovyeva, L.S. (1996) ‘The structure of theskulls of caudate amphibians collected from the Jurassic of Kirgizstan and the Cretaceousof Uzbekistan’, Vestnik Sankt-Petersburgiskogo Universiteta, Geologia, 7: 3–11.

Nixon, K.C. and Davis, J.I. (1991) ‘Polymorphic taxa, missing values and cladistic analysis’,Cladistics, 7: 233–41.

Panchen, A.L. (1985) ‘On the amphibian Crassigyrinus scoticus Watson from theCarboniferous of Scotland’, Philosophical Transactions of the Royal Society, London,B309: 505–68.

Panchen, A.L. and Smithson, T.R. (1987) ‘Character diagnosis, fossils, and the origin oftetrapods’, Biological Reviews, 62: 341–438.

—— (1988) ‘The relationships of the earliest tetrapods’, in M.J. Benton (ed.) The Phylogenyand Classification of the Tetrapods, 1: Amphibians, Reptiles, Birds, Oxford: Clarendon Press,pp. 1–32.

—— (1990) ‘The pelvic girdle and hind limb of Crassigyrinus scoticus (Lydekker) from the ScottishCarboniferous and the origin of the tetrapod pelvic skeleton’, Transactions of the Royal Societyof Edinburgh: Earth Sciences, 81: 31–44.

Paton, R.L., Smithson, T.R. and Clack, J.A. (1999) ‘An amniote-like skeleton from the EarlyCarboniferous of Scotland’, Nature, 398: 508–13.

Platz, J.E. and Conlon, J.M. (1997) ‘and turn back again’, Nature, 389: 246.Pough, F.H., Andrews, R.M., Cadle, J.E., Crump, M.L., Savitzky, A.H. and Wells, K.D.

(2000) Herpetology, 2nd edn, Upper Saddle River: Prentice Hall.Prendini, L. (2001) ‘Species or supraspecific taxa as terminals in cladistic analysis?

Groundplans versus exemplars revisited’, Systematic Biology, 50: 290–300.Rage, J.-C. and Rocek, Z. (1989) ‘Redescription of Triadobatrachus massinoti (Piveteau,

1936) an anuran amphibian from the early Triassic’, Palaeontographica Abteilung A, 206:1–16.

Reisz, R.R. and Laurin, M. (1991) ‘Owenetta and the origin of turtles’, Nature, 349: 324–6.Rieppel, O. (2000) ‘Turtles as diapsid reptiles’, Zoological Scripta, 29: 199–212.Rieppel, O. and deBraga, M. (1996) ‘Turtles as diapsid reptiles’, Nature, 384: 453–5.Rieppel, O. and Reisz, R.R. (1999) ‘The origin and early evolution of turtles’, Annual Review

of Ecology and Systematics, 30: 1–22.

TTEC11 05/06/2003 11:47 AM Page 256


Rocek, Z. and Rage, J.-C. (2000a) ‘Anatomical transformations in the transition from temno-spondyl to proanuran stages’, in H. Heatwole and R.L. Carroll (eds) Amphibian Biology, 4:Palaeontology, Chipping Norton: Surrey Beatty & Sons, pp. 1274–82.

—— (2000b) ‘Proanuran stages (Triadobatrachus, Czatkobatrachus)’, in H. Heatwole andR.L. Carroll (eds) Amphibian Biology, 4: Palaeontology, Chipping Norton: Surrey Beatty &Sons, pp. 1283–94.

Rolfe, W.D.I., Clarkson, E.N.K. and Panchen, A.L. (eds) (1994) Volcanism and EarlyTerrestrial Biotas, Transactions of the Royal Society of Edinburgh: Earth Sciences, 84: 175–464.

Romer, A.S. (1946) ‘The primitive reptile Limnoscelis restudied’, American Journal of Science,244: 149–88.

Ruta, M., Milner, A.R. and Coates, M.I. (2001) ‘The tetrapod Caerorhachis bairdi Holmesand Carroll from the Lower Carboniferous of Scotland’, Transactions of the Royal Societyof Edinburgh: Earth Sciences, 92: 229–61.

Ruta, M., Coates, M.I. and Quicke, D.L.J. (in press) ‘Early tetrapod relationships revisited’,Biological Reviews.

Salisbury, B.A. and Kim, J. (2001) ‘Ancestral state estimation and taxon sampling density’,Systematic Biology, 50: 557–64.

Schoch, R.R. (1992) ‘Comparative ontogeny of Early Permian branchiosaurid amphibians fromSouthwestern Germany’, Palaeontographica Abteilung A, 222: 43–83.

—— (1995) ‘Heterochrony in the development of the amphibian head’, in K.J. McNamara (ed.)Evolutionary Change and Heterochrony, New York: John Wiley & Sons, pp. 107–24.

—— (1998) ‘Homology of cranial ossifications in urodeles’, Neues Jahrbuch für Geologie undPaläontologie, Monatshefte, 1998: 1–25.

Schoch, R.R. and Milner, A.R. (2000) Handbuch der Palëoherpetologie: Teil 3B,Stereospondyli, Munich: Pfeil Verlag.

Shishkin, M.A. (1998) ‘Tungussogyrinus, a relict neotenic dissorophoid (Amphibia,Temnospondyli) from the Permo-Triassic of Siberia’, Palentologicheskij Zhurnal, 5: 521–31.

Smirnov, S. (1986) ‘The evolution of the urodele sound-conducting apparatus’, in Z. Rocek(ed.) Studies in Herpetology, Proceedings of the European Herpetological Meeting, Prague:Charles University, pp. 55–8.

Smith, A.B. (1994) Systematics and the Fossil Record, London: Blackwell Science.—— (1999) ‘Dating the origin of metazoan body plans’, Evolution and Development, 1:

138–42.Smithson, T.R. (1982) ‘The cranial morphology of Greererpeton burkemorani Romer

(Amphibia: Temnospondyli)’, Zoological Journal of the Linnean Society, 76: 29–90.—— (1985) ‘The morphology and relationships of the Carboniferous amphibian Eoherpeton

watsoni Panchen’, Zoological Journal of the Linnean Society, 85: 317–410.—— (1989) ‘The earliest known reptile’, Nature, 314: 676–8.—— (1994) ‘Eldeceeon rolfei, a new reptiliomorph from the Viséan of East Kirkton, West Lothian,

Scotland’, Transactions of the Royal Society of Edinburgh: Earth Sciences, 84: 377–82.—— (2000) ‘Anthracosaurs’, in H. Heatwole and R.L. Carroll (eds) Amphibian Biology, 4:

Palaeontology, Chipping Norton: Surrey Beatty & Sons, pp. 1053–64.Smithson, T.R. and Rolfe, W.D.I. (1990) ‘Westlothiana gen. nov.: naming the earliest known

reptile’, Scottish Journal of Geology, 26: 137–8.Smithson, T.R., Carroll, R.L., Panchen, A.L. and Andrews, S.M. (1994) ‘Westlothiana lizziae

from the Viséan of East Kirkton, West Lothian, Scotland, and the amniote stem’,Transactions of the Royal Society of Edinburgh: Earth Sciences, 84: 383–412.

Stauffer, S.L., Walker, A., Ryder, O.A., Lyons-Weiler, M. and Hedges, S.B. (2001) ‘Humanand ape molecular clocks and constraints on paleontological hypotheses’, Journal ofHeredity, 92: 469–74.

TTEC11 05/06/2003 11:47 AM Page 257


Sumida, S.S. (1997) ‘Locomotor features of taxa spanning the origin of amniotes’, in S.S. Sumidaand K.L.M. Martin (eds) Amniote Origins: Completing the Transition to Land, London:Academic Press, pp. 353–98.

Sumida, S.S. and Lombard, R.E. (1991) ‘The atlas-axis complex in the Late Paleozoic diadec-tomorph amphibian Diadectes and the characteristics of the atlas-axis complex across theamphibian to amniote transition’, Journal of Paleontology, 65: 973–83.

Sumida, S.S., Lombard, R.E. and Berman, D.S. (1992) ‘Morphology of the atlas-axis complexof the late Palaeozoic tetrapod suborders Diadectomorpha and Seymouriamorpha’,Philosophical Transactions of the Royal Society, London, B336: 259–73.

Swofford, D.L. (1998) PAUP*: Phylogenetic analysis using parsimony (*and other methods),Version 4.0b4a, Sunderland, Massachusetts: Sinauer Associates.

Thulborn, R.A., Warren, A.A., Turner, S. and Hamley, T. (1996) ‘Carboniferous tetrapods fromAustralia’, Nature, 381: 777–80.

Trueb, L. and Cloutier, R. (1991) ‘A phylogenetic investigation of the inter- and intrarelationshipsof the Lissamphibia (Amphibia: Temnospondyli)’, in H.-P. Schultze and L. Trueb (eds) Originsof the Higher Groups of Tetrapods: Controversy and Consensus, Ithaca: Cornell UniversityPress, pp. 223–313.

van Tuinen, M., Porder, S. and Hadly, E.A. (2001) ‘Putting confidence limits around molecularand fossil divergence dates’, Journal of Vertebrate Paleontology, 21 (3 – supplement): 110A.

Wagner, P.J. (2000) ‘Phylogenetic analyses and the fossil record: tests and inferences, hypo-theses and models’, in D.H. Erwin and S.L. Wing (eds) Deep Time: Paleobiology’s Perspect-ive, Lawrence: The Paleontological Society, pp. 341–71.

Wellstead, C.F. (1982) ‘A Lower Carboniferous aïstopod amphibian from Scotland’,Palaeontology, 25: 193–208.

—— (1991) ‘Taxonomic revision of the Lysorophia, Permo-Carboniferous lepospondylamphibians’, Bulletin of the American Museum of Natural History, 209: 1–90.

Wilkinson, M. (1995) ‘Coping with abundant missing entries in phylogenetic inference usingparsimony’, Systematic Biology, 44: 501–14.

Wood, S.P., Panchen, A.L. and Smithson, T.R. (1985) ‘A terrestrial fauna from the ScottishLower Carboniferous’, Nature, 314: 355–6.

Yates, A.M. and Warren, A.A. (2000) ‘The phylogeny of the “higher” temnospondyls(Vertebrata: Choanata) and its implications for the monophyly and origins of theStereospondyli’, Zoological Journal of the Linnean Society, 128: 77–121.

Yeh, J. (2002) ‘The effect of miniaturized body size on skeletal morphology in frogs’,Evolution, 56: 628–41.

Zardoya, R. and Meyer, A. (1998) ‘Complete mitochondrial genome suggests diapsid affinitiesof turtles’, Proceedings of the National Academy of Sciences, USA, 95: 14226–31.

Zhu, M. and Schultze, H.-P. (2001) ‘Inter-relationships of basal osteichthyans’, in P.E. Ahlberg(ed.) Major Events in Early Vertebrate Evolution: Palaeontology, Phylogeny, Genetics andDevelopment, London: Taylor & Francis, pp. 289–314.

Appendix 11.1

Ahlberg and Clack’s (1998) matrix of lower jaw characters is reproduced below. For conveni-ence, characters are divided into groups of five separated by a space. For a description of characters 1–50 (upper row), the reader is referred to their publication. Eleven new characters(51–61; lower row), typed in bold, are added. Caerorhachis (Holmes and Carroll 1977;Ruta et al. 2001) is included. As in the original analysis, question marks denote missing orinapplicable characters. Character 48 (postsplenial pit line) was changed from 1 (absent) to ?(inapplicable) in Diploceraspis, Sauropleura, Eocaptorhinus, and Ophiacodon (Ruta et al. 2001).The new characters are as follows:

TTEC11 05/06/2003 11:47 AM Page 258


51. Rearmost extension of mesial lamina of splenial closer to anterior margin of adductor fossa(0) than to anterior end of lower jaw (1).52. Lateral exposure of dentary smaller (0) or greater (1) than lateral exposure of angular.53. Absence (0) or presence (1) of at least one Meckelian foramen comparable in length withthe adductor fossa.54. Absence (0) or presence (1) of small posterior Meckelian foramen between prearticular andangular.55. Absence (0) or presence (1) of small posterior Meckelian foramen between prearticular,postsplenial, and angular.56. Absence (0) or presence (1) of intermediate Meckelian foramen between prearticular adpostsplenial.57. Absence (0) or presence (1) of condition: maximum depth of mesial lamina of splenial comparable with maximum depth of prearticular when both are measured at the level of themid-length of the adductor fossa.58. Absence (0) or presence (1) of retroarticular process.59. Absence (0) or presence (1) of condition: posterior coronoid exposed in lateral view.60. Absence (0) or presence (1) of condition: mesial lamina of angular deeper than prearticu-lar when both are measured at the level of the anterior margin of the adductor fossa.61. Absence (0) or presence (1) of condition: mesial margin of posterior coronoid shorter thanthat of mid-coronoid and up to about two-thirds as long as the latter.

Panderichthys00000 00000 00000 0000? 00000 00000 00000 00000 00000 00000 000?0 0000? 0

Elginerpeton??00? 0?000 01111 00100 01010 00111 11000 ?1??0 01??0 0001? ?00?0 0000? ?

Obruchevichthys????? 0?00? 0?0?? 00101 01??? ?0111 11??? ????0 ??0?0 1??1? ????0 0???? ?

Ichthyostega00001 00001 11111 ??001 01000 00011 ??000 01000 1?000 00010 100?? ?000? 0

Ventastega00001 00001 11001 11001 01000 00111 10000 01000 01000 00010 ?00?? ?000? 0

Metaxygnathus10000 00001 ?1010 11001 0101? 00?11 11000 00000 01000 10?10 000?? ?000? ?

Acanthostega10000 00000 11111 11001 01101 00010 11000 01000 01000 10110 000?? ?000? 0

Whatcheeria?0?0? ?0001 1111? 00001 01?01 00001 100?? ?10?? ????0 2011? ?00?? ??00? ?

Tulerpeton?00?? ??000 ?1?1? ?0?01 0???? ????? ????? ????? ????0 30?1? ????? ????? ?

Crassigyrinus00100 00131 11111 10011 01101 00?01 01000 1??00 0?100 30110 1000? ?0000 0

TTEC11 05/06/2003 11:47 AM Page 259


Greererpeton00111 00121 11011 00001 01101 10001 01000 ?0000 1?100 20110 1000? ?0000 0

Megalocephalus00101 10031 11111 00011 01102 00001 01011 10010 1?100 30111 10000 00000 0

Pholiderpeton00101 11131 11111 00001 01101 10001 00010 ?1110 1?110 30111 10101 01011 0

Cochleosaurus00101 11131 11111 00001 01101 1100? ??011 20010 1?111 ??111 000?? ?00?0 ?

Phonerpeton00101 11131 11111 00001 01101 0100? ??011 ?0111 1?111 ??111 00000 00010 0

Eoherpeton00101 1?111 11111 00001 ?1101 0???? ??010 ?01?0 0?111 ??111 10001 10000 1

Proterogyrinus001?1 ?0131 11111 00001 01101 1???? ??0?0 ?11?? 1?111 ??111 1010? ?1001 0

Gephyrostegus00101 11111 11??1 00001 01101 1100? ??000 201?0 1?111 ??111 1010? ?1010 0

Balanerpeton00101 0?131 11111 00001 01102 0100? ??011 ?0001 1?101 ??111 00000 00000 0

Platyrhinops0?101 01131 11111 00001 01101 0100? ??011 ?0??1 1?101 ??111 ??0?? ?00?0 ?

Microbrachis01101 01131 11111 00001 01101 0100? ??011 ?0000 1?110 31111 10010 00000 0

Discosauriscus01101 11131 11111 00001 11101 0100? ??011 ?01?0 1?110 31111 11010 00010 0

Eocaptorhinus0112? ?1031 ????1 00001 11101 0100? ??1?? ?0000 1?111 ???11 1100? ?10?1 ?

Diploceraspis0112? ?00?1 11111 00001 11101 0100? ??1?? ?0000 1?111 ???01 1001? ?0100 ?

Sauropleura011?? ????? ????? 00001 11101 11??? ??1?? ?0000 01111 ???11 1010? ?0101 ?

Ophiacodon00111 ?1031 1?1?1 00001 11101 0100? ??1?? ?0100 1?111 ????1 1100? ?100? ?

Caerorhachis01101 01131 11111 ???11 01101 00001 01010 ?0010 00111 ???01 1?001 10000 1

TTEC11 05/06/2003 11:47 AM Page 260


Discosauriscus

Megalocephalus

OphiacodonEocaptorhinusSauropleuraDiploceraspisDiscosauriscusMicrobrachisProterogyrinusPholiderpetonGephyrostegusCaerorhachisEoherpetonPlatyrhinopsBalanerpetonPhonerpetonCochleosaurusMegalocephalusCrassigyrinusGreererpetonWhatcheeriaIchthyostegaAcanthostegaMetaxygnathusVentastegaTulerpetonObruchevichthysElginerpetonPanderichthys

AEocaptorhinus

Discosauriscus

Cochleosaurus

MegalocephalusCrassigyrinus

AcanthostegaMetaxygnathus

Obruchevichthys

Panderichthys

ProterogyrinusPholiderpetonGephyrostegus

Ophiacodon

SauropleuraDiploceraspis

MicrobrachisPlatyrhinopsBalanerpetonPhonerpeton

GreererpetonWhatcheeriaIchthyostega

VentastegaTulerpeton

Elginerpeton

CaerorhachisEoherpeton

B

OphiacodonEocaptorhinusSauropleuraDiploceraspisDiscosauriscusMicrobrachisPlatyrhinopsBalanerpetonPhonerpetonCochleosaurus

Megalocephalus

CrassigyrinusGreererpeton

Whatcheeria

Ichthyostega

AcanthostegaMetaxygnathusVentastega

Tulerpeton

ObruchevichthysElginerpetonPanderichthys

ProterogyrinusPholiderpetonGephyrostegusCaerorhachisEoherpeton

DOphiacodonEocaptorhinusSauropleuraDiploceraspis

MicrobrachisProterogyrinusPholiderpetonGephyrostegusCaerorhachisEoherpetonPlatyrhinopsBalanerpetonPhonerpetonCochleosaurus

CrassigyrinusGreererpeton

Whatcheeria

Ichthyostega

AcanthostegaMetaxygnathusVentastega

Tulerpeton

ObruchevichthysElginerpetonPanderichthys

C

Figure 11.11 A strict consensus of 25 fundamental trees derived from the analysis of an expandedversion of Ahlberg and Clack’s (1998) dataset with all characters unweighted andunordered; B, strict consensus of five fundamental trees obtained after reweightingcharacters by their rescaled consistency indices values (best fit); C, strict consensusof five fundamental trees obtained when characters 36 and 46 of Ahlberg and Clack(1998) were ordered, but leaving all characters unweighted; D, single tree foundafter reweighting characters by their rescaled consistency indices values (best fit), withcharacters 36 and 46 ordered.

TTEC11 05/06/2003 11:47 AM Page 261


The data matrix was processed with PAUP*4.0b10 under the following search settings: 1000random stepwise additions with one tree held in memory at any one time (MAXTREES = 1),followed by TBR branch-swapping (MAXTREES = unlimited) on trees in memory. The initialrun, with all characters unordered and equally weighted, yielded 25 equally parsimonious treesat 139 steps (CI = 0.4779; RI = 0.694; RC = 0.3395), a strict consensus of which is shownin Figure 11.11A. Reweighting characters by the maximum value of their rescaled consistencyindices gives five trees (CI = 0.6722; RI = 0.849; RC = 0.5884). The strict consensus of these(Figure 11.11B) has been used to construct Figure 11.4.

If characters 36 (position of centre of radiation on prearticular) and 46 (enclosure ofmandibular canal) are ordered as in Ahlberg and Clack’s (1998) analysis, then a PAUP* runwith all characters equally weighted gives five most parsimonious trees at 140 steps (CI = 0.4745;RI = 0.7025; RC = 0.3412), the strict consensus of which is illustrated in Figure 11.11C. Ifcharacters are reweighted by the maximum value of their rescaled consistency indices, then asingle tree is obtained (Figure 11.11D; CI = 0.6772; RI = 0.8641; RC = 0.603).

TTEC11 05/06/2003 11:47 AM Page 262

Bones, molecules, and crown- tetrapod origins...Schultze 2001). The monophyletic status of the...

Documents

Transcript of Bones, molecules, and crown- tetrapod origins...Schultze 2001). The monophyletic status of the...